CAIRO — As protesters in Tahrir Square faced off against pro-government forces, they drew a lesson from their counterparts in Tunisia: “Advice to the youth of Egypt: Put vinegar or onion under your scarf for tear gas.”

The exchange on Facebook was part of a remarkable two-year collaboration that has given birth to a new force in the Arab world — a pan-Arab youth movement dedicated to spreading democracy in a region without it. Young Egyptian and Tunisian activists brainstormed on the use of technology to evade surveillance, commiserated about torture and traded practical tips on how to stand up to rubber bullets and organize barricades.

They fused their secular expertise in social networks with a discipline culled from religious movements and combined the energy of soccer fans with the sophistication of surgeons. Breaking free from older veterans of the Arab political opposition, they relied on tactics of nonviolent resistance channeled from an American scholar through a Serbian youth brigade — but also on marketing tactics borrowed from Silicon Valley.

As their swelling protests shook the Egyptian state, they were locked in a virtual tug of war with a leader with a very different vision — Gamal Mubarak, the son of President Hosni Mubarak, a wealthy investment banker and ruling-party power broker. Considered the heir apparent to his father until the youth revolt eliminated any thought of dynastic succession, the younger Mubarak pushed his father to hold on to power even after his top generals and the prime minister were urging an exit, according to American officials who tracked Hosni Mubarak’s final days.

The defiant tone of the president’s speech on Thursday, the officials said, was largely his son’s work.

“He was probably more strident than his father was,” said one American official, who characterized Gamal’s role as “sugarcoating what was for Mubarak a disastrous situation.” But the speech backfired, prompting Egypt’s military to force the president out and assert control of what they promise will be a transition to civilian government.

Now the young leaders are looking beyond Egypt. “Tunis is the force that pushed Egypt, but what Egypt did will be the force that will push the world,” said Walid Rachid, one of the members of the April 6 Youth Movement, which helped organize the Jan. 25 protests that set off the uprising. He spoke at a meeting on Sunday night where the members discussed sharing their experiences with similar youth movements in Libya, Algeria, Morocco and Iran.

“If a small group of people in every Arab country went out and persevered as we did, then that would be the end of all the regimes,” he said, joking that the next Arab summit might be “a coming-out party” for all the ascendant youth leaders.

Bloggers Lead the Way

The Egyptian revolt was years in the making. Ahmed Maher, a 30-year-old civil engineer and a leading organizer of the April 6 Youth Movement, first became engaged in a political movement known as Kefaya, or Enough, in about 2005. Mr. Maher and others organized their own brigade, Youth for Change. But they could not muster enough followers; arrests decimated their leadership ranks, and many of those left became mired in the timid, legally recognized opposition parties. “What destroyed the movement was the old parties,” said Mr. Maher, who has since been arrested four times.

By 2008, many of the young organizers had retreated to their computer keyboards and turned into bloggers, attempting to raise support for a wave of isolated labor strikes set off by government privatizations and runaway inflation.

After a strike that March in the city of Mahalla, Egypt, Mr. Maher and his friends called for a nationwide general strike for April 6. To promote it, they set up a Facebook group that became the nexus of their movement, which they were determined to keep independent from any of the established political groups. Bad weather turned the strike into a nonevent in most places, but in Mahalla a demonstration by the workers’ families led to a violent police crackdown — the first major labor confrontation in years.

Just a few months later, after a strike in Tunisia, a group of young online organizers followed the same model, setting up what became the Progressive Youth of Tunisia. The organizers in both countries began exchanging their experiences over Facebook. The Tunisians faced a more pervasive police state than the Egyptians, with less latitude for blogging or press freedom, but their trade unions were stronger and more independent. “We shared our experience with strikes and blogging,” Mr. Maher recalled.

For their part, Mr. Maher and his colleagues began reading about nonviolent struggles. They were especially drawn to a Serbian youth movement called Otpor, which had helped topple the dictator Slobodan Milosevic by drawing on the ideas of an American political thinker, Gene Sharp. The hallmark of Mr. Sharp’s work is well-tailored to Mr. Mubark’s Egypt: He argues that nonviolence is a singularly effective way to undermine police states that might cite violent resistance to justify repression in the name of stability.

The April 6 Youth Movement modeled its logo — a vaguely Soviet looking red and white clenched fist—after Otpor’s, and some of its members traveled to Serbia to meet with Otpor activists.

Another influence, several said, was a group of Egyptian expatriates in their 30s who set up an organization in Qatar called the Academy of Change, which promotes ideas drawn in part on Mr. Sharp’s work. One of the group’s organizers, Hisham Morsy, was arrested during the Cairo protests and remained in detention.

“The Academy of Change is sort of like Karl Marx, and we are like Lenin,” said Basem Fathy, another organizer who sometimes works with the April 6 Youth Movement and is also the project director at the Egyptian Democratic Academy, which receives grants from the United States and focuses on human rights and election-monitoring. During the protesters’ occupation of Tahrir Square, he said, he used his connections to raise about $5,100 from Egyptian businessmen to buy blankets and tents.

‘This Is Your Country’

Then, about a year ago, the growing Egyptian youth movement acquired a strategic ally, Wael Ghonim, a 31-year-old Google marketing executive. Like many others, he was introduced into the informal network of young organizers by the movement that came together around Mohamed ElBaradei, the Nobel Prize-winning diplomat who returned to Egypt a year ago to try to jump-start its moribund political opposition.

Mr. Ghonim had little experience in politics but an intense dislike for the abusive Egyptian police, the mainstay of the government’s power. He offered his business savvy to the cause. “I worked in marketing, and I knew that if you build a brand you can get people to trust the brand,” he said.

The result was a Facebook group Mr. Ghonim set up: We Are All Khalid Said, after a young Egyptian who was beaten to death by police. Mr. Ghonim — unknown to the public, but working closely with Mr. Maher of the April 6 Youth Movement and a contact from Mr. ElBaradei’s group — said that he used Mr. Said’s killing to educate Egyptians about democracy movements.

He filled the site with video clips and newspaper articles about police violence. He repeatedly hammered home a simple message: “This is your country; a government official is your employee who gets his salary from your tax money, and you have your rights.” He took special aim at the distortions of the official media, because when the people “distrust the media then you know you are not going to lose them,” he said.

He eventually attracted hundreds of thousands of users, building their allegiance through exercises in online democratic participation. When organizers planned a “day of silence” in the Cairo streets, for example, he polled users on what color shirts they should all wear — black or white. (When the revolt exploded, the Mubarak government detained him for 12 days in blindfolded isolation in a belated attempt to stop his work.)

After the Tunisian revolution on Jan. 14, the April 6 Youth Movement saw an opportunity to turn its little-noticed annual protest on Police Day — the Jan. 25 holiday that celebrates a police revolt that was suppressed by the British — into a much bigger event. Mr. Ghonim used the Facebook site to mobilize support. If at least 50,000 people committed to turn out that day, the site suggested, the protest could be held. More than 100,000 signed up.

“I have never seen a revolution that was preannounced before,” Mr. Ghonim said.

By then, the April 6 movement had teamed up with Mr. ElBaradei’s supporters, some liberal and leftist parties, and the youth wing of the Muslim Brotherhood to plaster Cairo with eye-catching modernist posters advertising their Tunisia-inspired Police Day protest. But their elders — even members of the Brotherhood who had long been portrayed as extremists by Mr. Mubarak and the West — shied away from taking to the streets.

Explaining that Police Day was supposed to honor the fight against British colonialism, Essem Erian, a Brotherhood leader, said, “On that day we should all be celebrating together.

“All these people are on Facebook, but do we know who they are?” he asked. “We cannot tie our parties and entities to a virtual world.”

‘This Was It’

When the 25th came, the coalition of young activists, almost all of them affluent, wanted to tap into the widespread frustration with the country’s autocracy, and also with the grinding poverty of Egyptian life. They started their day trying to rally poor people with complaints about pocketbook issues: “They are eating pigeon and chicken, but we eat beans every day.”

By the end of the day, when tens of thousands had marched to Tahrir Square, their chants had become more sweeping. “The people want to bring down the regime,” they shouted, a slogan that the organizers said they had read in signs and on Facebook pages from Tunisia. Mr. Maher of the April 6 Youth Movement said the organizers even debated storming Parliament and the state television building — classic revolutionary moves.

“When I looked around me and I saw all these unfamiliar faces in the protests, and they were more brave than us — I knew that this was it for the regime,” Mr. Maher said.

It was then that they began to rely on advice from Tunisia, Serbia and the Academy of Change, which had sent staff members to Cairo a week before to train the protest organizers. After the police used tear gas to break up the protest that Tuesday, the organizers came back better prepared for their next march on Friday, the 28th, the “Day of Rage.”

This time, they brought lemons, onions and vinegar to sniff for relief from the tear gas, and soda or milk to pour into their eyes. Some had fashioned cardboard or plastic bottles into makeshift armor worn under their clothes to protect against riot police bullets. They brought spray paint to cover the windshields of police cars, and they were ready to stuff the exhaust pipes and jam the wheels to render them useless. By the early afternoon, a few thousand protesters faced off against well over a thousand heavily armed riot police officers on the four-lane Kasr al-Nile Bridge in perhaps the most pivotal battle of the revolution.

“We pulled out all the tricks of the game — the Pepsi, the onion, the vinegar,” said Mr. Maher, who wore cardboard and plastic bottles under his sweater, a bike helmet on his head and a barrel-top shield on his arm. “The strategy was the people who were injured would go to the back and other people would replace them,” he said. “We just kept rotating.” After more than five hours of battle, they had finally won — and burned down the empty headquarters of the ruling party on their way to occupy Tahrir Square.

Pressuring Mubarak

In Washington that day, President Obama turned up, unexpectedly, at a 3:30 p.m. Situation Room meeting of his “principals,” the key members of the national security team, where he displaced Thomas E. Donilon, the national security adviser, from his seat at the head of the table.

The White House had been debating the likelihood of a domino effect since youth-driven revolts had toppled President Zine el-Abidine Ben Ali in Tunisia, even though the American intelligence community and Israel’s intelligence services had estimated that the risk to President Mubarak was low — less than 20 percent, some officials said.

According to senior officials who participated in Mr. Obama’s policy debates, the president took a different view. He made the point early on, a senior official said, that “this was a trend” that could spread to other authoritarian governments in the region, including in Iran. By the end of the 18-day uprising, by a White House count, there were 38 meetings with the president about Egypt. Mr. Obama said that this was a chance to create an alternative to “the Al Qaeda narrative” of Western interference.

American officials had seen no evidence of overtly anti-American or anti-Western sentiment. “When we saw people bringing their children to Tahrir Square, wanting to see history being made, we knew this was something different,” one official said.

On Jan. 28, the debate quickly turned to how to pressure Mr. Mubarak in private and in public — and whether Mr. Obama should appear on television urging change. Mr. Obama decided to call Mr. Mubarak, and several aides listened in on the line. Mr. Obama did not suggest that the 82-year-old leader step aside or transfer power. At this point, “the argument was that he really needed to do the reforms, and do them fast,” a senior official said. Mr. Mubarak resisted, saying the protests were about outside interference.

According to the official, Mr. Obama told him, “You have a large portion of your people who are not satisfied, and they won’t be until you make concrete political, social and economic reforms.”

The next day, the decision was made to send former Ambassador Frank G. Wisner to Cairo as an envoy. Mr. Obama began placing calls to Prime Minister Benjamin Netanyahu of Israel, Prime Minister Recep Tayyip Erdogan of Turkey and other regional leaders.

The most difficult calls, officials said, were with King Abdullah of Saudi Arabia and Mr. Netanyahu, who feared regional instability and urged the United States to stick with Mr. Mubarak. According to American officials, senior members of the government in Saudi Arabia argued that the United States should back Mr. Mubarak even if he used force against the demonstrators. By Feb. 1, when Mr. Mubarak broadcast a speech pledging that he would not run again and that elections would be held in September, Mr. Obama concluded that the Egyptian president still had not gotten the message.

Within an hour, Mr. Obama called Mr. Mubarak again in the toughest, and last, of their conversations. “He said if this transition process drags out for months, the protests will, too,” one of Mr. Obama’s aides said.

Mr. Mubarak told Mr. Obama that the protests would be over in a few days.

Mr. Obama ended the call, the official said, with these words: “I respect my elders. And you have been in politics for a very long time, Mr. President. But there are moments in history when just because things were the same way in the past doesn’t mean they will be that way in the future.”

The next day, heedless of Mr. Obama’s admonitions, Mr. Mubarak launched another attack against the protesters, many of whom had by then spent five nights camped out in Tahrir Square. By about 2:30 p.m., thousands of burly men loyal to Mr. Mubarak and armed with rocks, clubs and, eventually, improvised explosives had come crashing into the square.

The protesters — trying to stay true to the lessons they had learned from Gandhi, the Rev. Dr. Martin Luther King Jr. and Gene Sharp — tried for a time to avoid retaliating. A row of men stood silent as rocks rained down on them. An older man told a younger one to put down his stick.

But by 3:30 p.m., the battle was joined. A rhythmic din of stones on metal rang out as the protesters beat street lamps and fences to rally their troops.

The Muslim Brotherhood, after sitting out the first day, had reversed itself, issuing an order for all able-bodied men to join the occupation of Tahrir Square. They now took the lead. As a secret, illegal organization, the Brotherhood was accustomed to operating in a disciplined hierarchy. The group’s members helped the protesters divide into teams to organize their defense, several organizers said. One team broke the pavement into rocks, while another ferried the rocks to makeshift barricades along their perimeter and the third defended the front.

“The youth of the Muslim Brotherhood played a really big role,” Mr. Maher said. “But actually so did the soccer fans” of Egypt’s two leading teams. “These are always used to having confrontations with police at the stadiums,” he said.

Soldiers of the Egyptian military, evidently under orders to stay neutral, stood watching from behind the iron gates of the Egyptian Museum as the war of stone missiles and improvised bombs continued for 14 hours until about four in the morning.

Then, unable to break the protesters’ discipline or determination, the Mubarak forces resorted to guns, shooting 45 and killing 2, according to witnesses and doctors interviewed early that morning. The soldiers — perhaps following orders to prevent excessive bloodshed, perhaps acting on their own — finally intervened. They fired their machine guns into the ground and into the air, several witnesses said, scattering the Mubarak forces and leaving the protesters in unmolested control of the square, and by extension, the streets.

Once the military demonstrated it was unwilling to fire on its own citizens, the balance of power shifted. American officials urged the army to preserve its bond with the Egyptian people by sending top officers into the square to reassure the protesters, a step that further isolated Mr. Mubarak. But the Obama administration faltered in delivering its own message: Two days after the worst of the violence, Mr. Wisner publicly suggested that Mr. Mubarak had to be at the center of any change, and Secretary of State Hillary Rodham Clinton warned that any transition would take time. Other American officials suggested Mr. Mubarak might formally stay in office until his term ended next September. Then a four-day-long stalemate ensued, in which Mr. Mubarak refused to budge, and the protesters regained momentum.

On Thursday, Mr. Mubarak’s vice president, Omar Suleiman, was on the phone with Vice President Joseph R. Biden Jr. at 2 p.m. in Washington, the third time they had spoken in a week. The airwaves were filled with rumors that Mr. Mubarak was stepping down, and Mr. Suleiman told Mr. Biden that he was preparing to assume Mr. Mubarak’s powers. But as he spoke to Mr. Biden and other officials, Mr. Suleiman said that “certain powers” would remain with Mr. Mubarak, including the power to dissolve the Parliament and fire the cabinet. “The message from Suleiman was that he would be the de facto president,” one person involved in the call said.

But while Mr. Mubarak huddled with his son Gamal, the Obama administration was in the dark about how events would unfold, reduced to watching cable television to see what Mr. Mubarak would decide. What they heard on Thursday night was a drastically rewritten speech, delivered in the unbowed tone of the father of the country, with scarcely any mention of a presumably temporary “delegation” of his power.

It was that rambling, convoluted address that proved the final straw for the Egyptian military, now fairly certain that it would have Washington’s backing if it moved against Mr. Mubarak, American officials said. Mr. Mubarak’s generals ramped up the pressure that led him at last, without further comment, to relinquish his power.

“Eighty-five million people live in Egypt, and less than 1,000 people died in this revolution — most of them killed by the police,” said Mr. Ghonim, the Google executive. “It shows how civilized the Egyptian people are.” He added, “Now our nightmare is over. Now it is time to dream.”