At least five people have died and more than 400 injured in clashes in the Egyptian capital, Cairo, as unrest grows over a controversial draft constitution.
Rocks and petrol bombs were thrown as supporters of President Mohammed Morsi tried to break up an opposition sit-in outside the presidential palace on Wednesday.
The army has deployed tanks and armoured vehicles outside the building.
Opposition leaders have accused the Muslim Brotherhood of organising the violence.
The government says a referendum on the constitution will go ahead next month, the BBC reports.
Speaking earlier on Wednesday, Vice-President Mahmoud Mekki said the vote was still scheduled for 15 December, but that the "door for dialogue" remained open, indicating that changes could be made to the document later.
Critics say the draft was rushed through parliament without proper consultation and that it does not do enough to protect political and religious freedoms and the rights of women.
The draft added to the anger generated by Mr Morsi passing a decree in late November which granted him wide-ranging new powers.
Four of Mr Morsi's advisers resigned on Wednesday in an apparent protest. Three others had done so last week.
Egypt's opposition coalition blamed Mr Morsi for the violence and said it was ready for dialogue if the Islamist leader scrapped a decree he issued on 22 November that gave him wide powers and shielded his decisions from judicial review.