Thousands spent the night in iconic Tahrir Square, focus of protests which brought down ex-leader Hosni Mubarak, to wait for thousands of other protesters for the massive Sunday demonstration.

Anti-Morsi supporters are chanting: "Irhal! Irhal!" ("Leave! Leave!") in Cairo. Protesters plan to convene mass demonstrations around the presidential palace by the evening.

In Alexandria, the second-biggest city, thousands of protesters gathered for a march to the central Sidi Gaber area.

Police are absent from the streets near Tahrir Square. Some police forces have publicly refused to protect Morsi and his backers in the Muslim Brotherhood.

A huge rally of presidential supporters is also under way in the Cairo suburb of Nasr City.

Tahrir Square is reportedly close to capacity despite the heat.

Interviewed by a British newspaper, Mursi repeated his determination to ride out what he sees as an undemocratic attack on his electoral legitimacy. But he also offered to revise the new constitution.

Some also seem to believe the army might force the president's hand. In Cairo, demonstrators stopped to shake hands and take photographs with soldiers guarding key buildings.