CAIRO — Egypt's capital was the scene of violent chaos Friday as tens of thousands of anti-government protesters stoned and confronted police, who fired back with rubber bullets and tear gas — a major escalation in the biggest challenge to President Hosni Mubarak's 30-year rule. Even a Nobel Peace laureate was soaked by water cannon and forced to take refuge in a mosque.

Large groups of protesters, in the thousands, gathered at at least six venues in Cairo, a city of about 18 million people, and many of them were on the move marching toward major squares and across Nile bridges. There were smaller protests in Assiut south of Cairo and al-Arish in the Sinai peninsula. Regional television stations were reporting clashes between thousands of demonstrators and police in the Mediterranean port city of Alexandria and Minya south of Cairo.

"It's time for this government to change," said Amal Ahmed, a 22-year-old protester. "I want a better future for me and my family when I get married."

Police fired water cannons at one of the country's leading pro-democracy advocates, Mohamed ElBaradei, and his supporters as they joined the latest wave of protests after noon prayers. They used batons to beat some of ElBaradei's supporters, who surrounded him to protect him.

A soaking wet ElBaradei was trapped inside a mosque while hundreds of riot police laid siege to it, firing tear gas in the streets around so no one could leave. Tear gas canisters set several cars ablaze outside the mosque and several people fainted and suffered burns.

Friday's protest were by far the largest and most violent since they began Tuesday. Demonstrators are demanding 82-year-old Mubarak's ouster and venting their rage at years of government neglect of rampant poverty, unemployment and rising food prices. The protesters have said they are emboldened by the uprising in Tunisia, another north African Arab nation. Egypt is Washington's closest Arab ally, but Mubarak may be losing U.S. support. The Obama administration has publicly counseled Mubarak to introduce reforms and refrain from using violence against the protesters.

President Barack Obama said Thursday the anti-government protests filling the streets show the frustrations of Egypt's citizens.

"It is very important that people have mechanisms in order to express their grievances," Obama said.

Authorities appeared to have disrupted social networking sites, used as an organizing tool by protesters, throughout the week. Those disruptions escalated overnight, when Internet and cell phone services, at least in Cairo, appeared to be largely cut off. However, the extreme measures did not prevent tens of thousands from flooding the streets.

In the upscale Mohandiseen neighborhood, at least 10,000 were marching toward the city center chanting "down, down with Mubarak." The crowd later swelled to about 20,000 as they made their way through residential areas.

Residents looking on from apartment block windows waved and whistled in support. Some waved the red, white and black Egyptian flags. The marchers were halted as they tried to cross a bridge over the Nile, when police fired dozens of tear gas canisters.

At Ramsis square in the heart of the city, thousands clashed with police as they left the al-Nur mosque after prayers. Police used tear gas and rubber bullets and some of the tear gas was fired inside the mosque where women were taking refuge. Hundreds later broke through police cordons to head to the main downtown square, Tahrir. But they were stopped by police firing tear gas.

Near Tahrir, hundreds of riot police in a cluster moved in, anticipating the arrival of large crowds. A short while later, thousands of protesters marched across a bridge over the Nile and moved toward the square, where police began firing tear gas at them.

Later, television footage showed a chaotic and violent scene where protesters were throwing rocks down on police from a highway overpass near Tahrir Square, while a police vehicle sped through the crowd spraying tear gas on demonstrators.

Clusters of riot police with helmets and shields were stationed around the city, at the entrances to bridges across the Nile and other key intersections.

Mubarak has not been seen publicly or heard from since the protests began Tuesday. While Mubarak may still have a chance to ride out this latest challenge, his choices are limited, and all are likely to lead to a loosening of his grip on power.

Mubarak has not said yet whether he will stand for another six-year term as president in elections this year. He has never appointed a deputy and is thought to be grooming his son Gamal to succeed him despite popular opposition. According to leaked U.S. memos, hereditary succession also does not meet with the approval of the powerful military.

Mubarak and his government have shown no hint of concessions to the protesters who want political reform and a solution to rampant poverty, unemployment and rising food prices.

Friday's demonstrations were energized by the return of Nobel Peace laureate ElBaradei on Thursday night, when he said he was ready to lead the opposition toward a regime change.

They also got a boost from the endorsement of the country's biggest opposition group, the Islamic fundamentalist Muslim Brotherhood. The group called its supporters to join the protests on Friday.

The Brotherhood, outlawed since 1954, is Egypt's largest and best organized opposition group. It seeks to establish an Islamic state. It renounced violence in the 1970s and has since been a peaceful movement. Its network of social and medical services has traditionally won it popular support, but its detractors say its involvement in politics has chipped away at its support base.

It made a surprisingly strong showing in 2005 parliamentary elections, winning 20 percent of the legislature's seats, but it failed to win a single seat in the latest election late last year. The vote is widely thought to have been marred, rigged to ensure that Mubarak's ruling party win all but a small fraction of the chamber's 518 seats.

Egypt's four primary Internet providers — Link Egypt, Vodafone/Raya, Telecom Egypt, Etisalat Misr — all stopped moving data in and out of the country at 12:34 a.m., according to a network security firm monitoring the traffic. Telecom experts said Egyptian authorities could have engineered the unprecedented cutoff with a simple change to the instructions for the companies' networking equipment.

The Internet appeared to remain cut off in Cairo but was restored in some smaller cities Friday morning. Cell-phone text and Blackberry Messenger services were all cut or operating sporadically in what appeared to be a move by authorities to disrupt the organization of demonstrations.

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Egyptians outside the country were posting updates on Twitter after getting information in voice calls from people inside the country. Many urged their friends to keep up the flow of information over the phones.

A Facebook page run by protesters listed their demands. They want Mubarak to declare that neither he nor his son will stand for next presidential elections; dissolve the parliament holds new elections; end to emergency laws giving police extensive powers of arrest and detention; release all prisoners including protesters and those who have been in jail for years without charge or trial; and immediately fire the interior minister.

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Associated Press reporter Hadeel Al-Shalchi contributed to this report from Cairo.