At least 59 people are reported to have been injured in fresh clashes between protesters and security forces near Egypt's defence ministry.
Security forces used water cannons and tear gas to drive back demonstrators.
A curfew has now been imposed in the area from 23:00 local time. Protesters have dispersed and some have joined a demonstration in Tahrir Square.
On Wednesday, unidentified assailants attacked people protesting outside the ministry, leaving at least 20 dead.
The protesters, who were demonstrating against the ruling Supreme Council of the Armed Forces (Scaf), accused the government of orchestrating the attack.
The BBC's Jon Leyne in Cairo said trouble flared on Friday when protesters ignored army warnings not to approach the ministry.
The unrest comes just three weeks before presidential elections are due to be held - the first since Hosni Mubarak was forced to step down.
Earlier on Friday, thousands once again gathered in Tahrir Square - the focus of the uprising that ousted President Mubarak in February 2011 - to protest against the Scaf.
The crowd expressed their anger at the generals' failure to protect the demonstration on Wednesday and reiterated the demand that they hand over power to a civilian administration immediately, rather than after the presidential election.
In the afternoon, protesters began walking from Tahrir Square to the defence ministry in the capital's Abbasiya district, several kilometres to the north-west.
The protesters started throwing stones and tried to break barbed-wire surrounding the building, sparking clashes with security forces.
TV pictures showed riot police and military police trying to disperse the crowd by firing water cannons and then tear gas. They could also be seen hurling back stones and beating some of the protesters with batons.
Soldiers broadcast a message on loud-hailers saying the defence ministry would only be stormed over their dead bodies, and that reinforcements were on the way.
State television blamed the Muslim Brotherhood for the trouble, despite the fact that the Islamist movement has been urging its supporters to stay away.
Clashes also happened between protesters and security forces in Egypt's second city of Alexandria on Friday.