Egypt was locked in a tense standoff on Monday after millions of protesters swarmed into the streets to demand the resignation of Islamist President Mohamed Morsi and militants set the ruling Muslim Brotherhood's headquarters on fire.
Young revolutionaries united with liberal and leftist opposition parties in a massive show of defiance on the first anniversary of Morsi's inauguration on Sunday, chanting "the people demand the fall of the regime".
The demonstrations, which brought half a million people to Cairo's central Tahrir Square and a similar crowd in the second city, Alexandria, were easily the largest since the Arab Spring uprising that ousted Hosni Mubarak in 2011.
Morsi, the most populous Arab state's first freely elected leader, stayed out of sight but acknowledged through a spokesman that he had made mistakes while adding that he was working to fix them and was open to dialogue. He showed no sign of quitting.
(Read More: Thousands in Egypt Protest Against President Morsi)
The massive protests showed that the ruling Muslim Brotherhood has not only alienated liberals and secularists by seeking to entrench Islamic rule but has also angered millions of ordinary Egyptians with economic mismanagement.
Tourism and investment have dried up, inflation is rampant and fuel supplies are running short, with power cuts lengthening in the summer heat.
Dozens of militants attacked the Brotherhood's national headquarters in Cairo with shotguns, petrol bombs and rocks, setting it on fire, and targeted offices of its political party across the country.
There was no sign of police or fire service protection for the Brotherhood's head office, where witnesses said guards inside the building fired on the attackers. Two people died and 11 were injured in that clash, hospital sources said.
Protest organisers called on Egyptians to keep occupying central squares across the country in a campaign of peaceful civil disobedience until Morsi quits.
Tens of thousands of demonstrators stayed in Tahrir Square long after midnight, appearing to heed the call for a sit-in.
Spotlight on Army
That put the spotlight on the army, which displayed its neutrality on Sunday, making goodwill gestures to the protesters after urging feuding politicians last week to cooperate to solve the nation's problems.
Some uniformed policemen marched among protesters in Cairo and Alexandria, chanting "the police and the people are one," and several senior officers addressed the Tahrir Square crowd.