Thousands of Egyptian anti-government protesters have clashed with police, who fired rubber bullets into the crowds and used tear gas and water cannons to disperse them.
The Cairo protest 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 Mr ElBaradei's supporters, who surrounded him to protect him. A soaking wet Mr ElBaradei was trapped inside a mosque nearly an hour after he and his supporters were drenched with water cannons.
Egyptian security officials said later Mr ElBaradei had been placed under house arrest.
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 Mr Mubarak's removal.
There were smaller protests in Assiut south of Cairo and al-Arish in the Sinai peninsula. Regional television stations also reported clashes between thousands of demonstrators and police in the Mediterranean port city of Alexandria and Minya south of Cairo.
Internet and mobile 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.
Mr Mubarak is the US'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.
Friday's demonstrations were energised by the return of Nobel Peace prize winner Mr ElBaradei on Thursday night, when he said he was ready to lead the opposition toward a regime change.