Protesters filled Tahrir Square in Cairo on Friday to denounce a move by Egypt's ruling generals to seize sweeping powers.
Islamist, secular and youth groups backed a call by the Muslim Brotherhood for protests across the country.
Last Friday the military issued decrees dissolving parliament and claiming all legislative power.
Results of presidential elections last weekend are still awaited.
The Muslim Brotherhood's Freedom and Justice Party announced on Tuesday that its candidate Mohammed Mursi, won with 51.74% of the vote.
His opponent, former Prime Minister Ahmed Shafiq, is also confident of victory.
But the Supreme Council for the Armed Forces says its decision was because of political "division and confusion" due the early release of unofficial results from the election.
Two decrees have been issued by the junta:
The first ordered parliament to be dissolved after a Supreme Constitutional Court ruling that the law on elections to the lower house was invalid because party members had been allowed to contest seats reserved for independents.
The second decree gave the generals complete control over legislation and military affairs until fresh parliamentary elections are held.