Egyptian opposition protesters have broken through a barricade erected by security forces to prevent them reaching Cairo’s presidential palace.
Thousands had marched on the palace in the capital after rejecting a call for dialogue by President Mohammed Morsi.
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.
A senior official later said Morsi could postpone the vote.
Vice-President Mahmud Mekki said Morsi could delay the 15 December plebiscite if the opposition guaranteed it would not challenge the move on the grounds that referendums legally had to be held two weeks after being formally presented to the president.
In a separate development, the election commission postponed the planned voting for Egyptians living abroad.
It said the voting – had been due to begin on Saturday – would now begin on Wednesday at the request of the foreign ministry.
Earlier on Friday, supporters of Morsi held their own march in the capital.
They vowed vengeance at a funeral for men killed in clashes earlier in the week.
However, opponents of the president gathered near the presidential palace. Some cut through barbed wire and surged up to the walls of the palace, Reuters news agency reported.
The president has been under fire over a decree he issued last month granting himself new powers.
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 spiraling crisis.
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.
“We [want] a dialogue not based on an arm-twisting policy and imposing fait accompli,” his message read.
Two other opposition groups, the liberal Wafd party and the National Association for Change, said they were also boycotting the talks.
The president angered his opponents on Thursday when he refused in a televised statement to withdraw his new powers and delay a referendum on Egypt’s draft constitution.
Earlier this week the presidential palace was the scene of bloody clashes, in which five people died and hundreds were injured.
Late on Thursday, opposition supporters ransacked the Muslim Brotherhood’s Cairo headquarters and set it on fire.
On Friday rival protesters between supporters and opponents of the president were reported in a number of Egyptian cities, including Alexandria in the north and Asyut in the south.
Morsi confirmed that the referendum on a new constitution would go ahead as planned, saying that if the constitution were voted down, another constituent assembly would be formed to write a new draft.
M.D