Thousands of Egyptian anti-government protesters clashed with police in Cairo, who fired rubber bullets into the crowds and used tear gas and water cannons to disperse them.
It was a major escalation in what was already the biggest challenge to President Hosni Mubarak's 30 year-rule.
Water cannons were used against pro-democracy leader Mohamed ElBaradei and his supporters as they joined the latest wave of protests after noon prayers.
Police also used batons to beat some of ElBaradei's supporters, who surrounded him to protect him.
A soaking wet ElBaradei was trapped inside a mosque nearly an hour after he and his supporters were drenched with water cannons.
Hundreds of riot police laid siege to the mosque, firing tear gas in the streets surrounding it so no one could leave. The tear gas canisters set several cars ablaze outside the mosque. Several people fainted and suffered burns.
Large groups of protesters, in the thousands, were gathered at at least six venues in Cairo, a city of about 18 million people. They are demanding Mubarak's removal.
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.
At the upmarket Mohandiseen district, at least 10,000 of people were marching toward the city centre 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 at them and whistled in support. Others waved the red, white and black Egyptian flags.
At Ramsis square in the heart of the city, thousands of protesters 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.
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.
Near the city's main Tahrir Square, hundreds of riot police clustered together and moved in, anticipating the arrival of a large crowd of protesters. 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 into the crowds.
Internet and cell-phone services were disrupted across Egypt starting overnight and throughout the day as authorities used extreme measures to hamper protesters from organising the mass rallies called after Friday prayers.
Mubarak is Washington's closest Arab ally, but Washington has signalled that he no longer enjoys its full backing, publicly advising him to introduce reform and refrain from using violence against the protesters. He has not been seen publicly or heard from since the protests began on Tuesday.
Today's demonstrations were energised by the return of Nobel Peace prize winner 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.