Opposition protesters in Cairo have broken through a barricade erected by security forces to prevent them reaching the presidential palace.
The BBC reports they marched on the palace in the capital after rejecting a call for dialogue by President Mohammed Mursi.
The opposition said the president had offered no concession on his decision to expand his powers and put a new draft constitution to a referendum on 15 December.
Opposition leaders reject that, saying Mr Mursi must rescind the decree before any talks begin.
A senior official later said Mr Morsi could conditionally postpone the vote.
Under Egyptian law, referendums must be held two weeks after being formally presented to the president.
However, Vice-President Mahmud Mekki said Mr Mursi could delay the referendum if the opposition agreed not to challenge the move on those grounds later.
On Thursday, there were clashes outside the presidential palace in which five people died.
Late on Thursday, opposition supporters ransacked the headquarters of the Muslim Brotherhood in Cairo and set it on fire.
Protests were also reported in a number of cities, including Alexandria in the north and Asyut in the south.
