An Egyptian woman flees as Egyptian anti-riot policemen clash with protesters today in Cairo, Egypt. Egyptian activists protested for a fourth day as social networking sites called for a mass rally in Cairo after Friday prayers.

CAIRO — Egypt’s capital was the scene of violent chaos today 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.

Groups of thousands of protesters, some chanting “out, out, out,” gathered at 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, Nobel Peace laureate 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.

“We are the ones who will bring change,” said 21-year-old Ahmed Sharif, one of scores who were with ElBardei. “If we do nothing, things will get worse. Change must come,” he screamed through a surgical mask he wore to ward off the tear gas.

Abeer Ahmed, a 31-year-old woman who showed up for ElBaradei with her toddler, said she has a law degree but makes a living cleaning homes.

“Nothing good is left in the country,” she said. “Oppression is growing.”