More protests are expected in Cairo later amid growing tension over the delay in announcing the result of the presidential election and claims that the military is trying to rig the outcome of the vote.
Thousands of supporters of the Muslim Brotherhood have been demonstrating in Tahrir Square since the polls closed on Sunday, demanding that their candidate, Mohamed Morsi, is declared the winner.
But his rival Ahmed Shafiq, Hosni Mubarak's former Prime Minister, is also claiming victory.
In his first public appearance since the election, Mr Shafiq said he was confident that he would be declared the "legitimate winner". The 70-year-old, derided by critics as a "remnant" of the old regime, criticised the Muslim Brotherhood for trying to put pressure on the country's election committee.
The final result of the election was due to have been announced on Thursday but election officials delayed the declaration until Sunday, saying they needed more time to consider 400 claims of voting irregularities.
The postponement has raised accusations from the Muslim Brotherhood's supporters that the military are trying to prevent the Islamist movement from taking power.
"They are trying to steal the election," one protester said in Tahrir Square. "We will stay here until Mohamed Morsi is declared our President," another said.
The ruling military council is meant to hand over power to the new president at the end of the month. But, adding to the suspicion that the generals are trying to secure their grip on Egypt, the military has granted itself control over lawmaking and the writing of a new constitution.
The Islamist dominated parliament has been dissolved on the orders of the military following a court order declaring the parliamentary election unconstitutional.
The initial results of the Presidential election suggested the Muslim Brotherhood's Mohamed Morsi was leading Mr Shafiq - viewed as the favoured candidate of the military - by almost a million votes.