Egyptian opposition leaders have rejected calls by President Mohammed Morsi to enter a national dialogue.
Mr Morsi, under fire for assuming broad new powers, had invited political groups for talks on Saturday.
But the opposition said the president had offered few concessions, and criticised his refusal to delay a constitutional referendum.
Both opponents and supporters of President Morsi have held fresh demonstrations in Cairo.
Opposition protesters marched on the presidential palace - the scene of deadly clashes in recent days. Some activists broke through police barricades around the palace, state TV reported.
Meanwhile thousands of Morsi supporters marched during the funerals of two men killed in the recent clashes.
The main opposition movement said on Friday it would not take part in Saturday's talks.
"The National Salvation Front is not taking part in the dialogue, that is the official stance," spokesman Ahmed Said confirmed in a statement.
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, state media report.
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. The vote is due to be held on 15 December.
Opposition leaders said Mr Morsi had missed a historic chance for compromise.
Egypt has been plunged into crisis since he issued a decree on 22 November stripping the judiciary of any power to challenge his decisions.
The BBC's Jon Leyne in Cairo says the growing tensions reveal deep divisions within the Muslim Brotherhood.
Whenever there is talk of compromise, the movement's hardliners seem to win the battle, our correspondent reports. Critics say Mr Morsi is not acting as president of the whole country, but rather as a delegate of the Muslim Brotherhood.
New clashes have been reported on Friday between supporters and opponents of President Morsi outside a mosque in the city of Alexandria.
The April 6 movement, an activist group that played a major role in last year's revolt against former President Hosni Mubarak, said on its Facebook page that protests on Friday would deliver a "red card" to Mr Morsi.
Other opposition groups also called for protests after Friday prayers across Egypt.
Earlier this week, thousands of protesters fought outside the presidential palace using stones, petrol bombs and guns. Five people were killed and hundreds more injured.
Late on Thursday, opposition supporters ransacked the Muslim Brotherhood's Cairo headquarters and set it on fire. The Muslim Brotherhood dominates the government and backs Mr Morsi.
Police also fired tear gas to disperse hundreds of protesters gathered outside the president's house in his hometown of Zagazig, north of Cairo.
Meanwhile, US President Barack Obama called Mr Morsi to express his "deep concern" over the recent violent protests, the White House said.
He welcomed Mr Morsi's call for talks, but stressed they should be "without preconditions", a statement said.
Mr Morsi has confirmed that the referendum on a new constitution will go ahead as planned, saying that if the constitution were voted down, another constituent assembly would be formed to write a new draft.
Critics say the draft, drawn up by a body dominated by Morsi-supporting Islamists, was rushed through parliament without proper consultation and does not do enough to protect political and religious freedoms and the rights of women.
On Thursday TV host Khayri Ramadan resigned on air, after the privately owned CBC channel reportedly refused to allow an opposition leader to appear on his show.
"Out of respect for my professionalism and for my dear guest who honoured me tonight, I announce my resignation," Mr Ramadan said.