Clashes have erupted between protesters and police in the Egyptian capital Cairo and two other cities.
Two people have been killed and hundreds wounded.
In scenes reminiscent of the 18-day uprising that toppled President Hosni Mubarak in February, hundreds of youths fought with riot police, who fired rubber bullets and tear gas.
Rocks were thrown and a police vehicle was set on fire.
The BBC reports Friday's demonstration was the largest in the square for months. About 50,000 people were there.
It's the biggest security challenge yet for the country's ruling generals days before scheduled elections.
The vote is being overshadowed by a row between political parties and the government over ground rules for a draft constitution that could leave the army free of civilian control.
Parliamentary elections are scheduled to begin on 28 November, the first since the removal of Mr Mubarak in February.