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.
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.