Egyptian opposition protesters have demonstrated outside the presidential palace in Cairo, after breaking through a barricade erected by security forces.
Tens of thousands had gathered near the palace after rejecting a call for dialogue by President Mohammed Morsi.
Opposition leaders say Mr Morsi has offered no concession on his decisions to expand his powers and to put a new draft constitution to a referendum.
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A top official later said the president 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 Morsi could delay the 15 December plebiscite if the opposition guaranteed agreed not to challenge the move on those grounds later, AFP news agency reports.
In a separate development, the election commission postponed the planned voting for Egyptians living abroad.
It said the voting - which had been due to begin on Saturday - would now begin on Wednesday at the request of the foreign ministry.
Earlier on Friday liberal and secularist opponents of the president gathered near the presidential palace.
They then cut through barbed wire and surged up to the outer walls of the palace, where many sprayed graffiti.
Meanwhile supporters of Mr Morsi held their own march in the capital, vowing vengeance at a funeral for men killed in clashes earlier in the week.
"Egypt is Islamic, it will not be secular, it will not be liberal!" the crowd chanted.
Nobel prize winner Mohamed ElBaradei, the movement's chief co-ordinator, posted a message on his Twitter account calling on political groups to shun all dialogue with Mr Morsi.
The main opposition movement, the National Salvation Front,
said it would not take part in talks Mr Morsi had offered to hold on
Saturday, in an effort to resolve the spiralling crisis. - BBC