Egypt was locked in a tense standoff on Monday after a day of huge rallies in which at least ten people were killed. The protesters demand the resignation of Islamist President Mohammed Morsi and some of them are saying that the military needs to step in, if Morsi refuses to step down.

The dead were all killed in Cairo or towns to its south. More than 600 were wounded during clashes between Morsi’s supporters and opponents, according to Al Arabiya reports.

Two men were killed by gunfire during an attack on the Muslim Brotherhood national headquarters in a suburb of the capital, medical sources said. Around 150 “thugs” attacked the building in the Moqqattam neighborhood with fire bombs, birdshot and stones, said a spokesman for the Muslim Brotherhood.

Half a million people crowded Cairo’s central Tahrir Square and a similar number protested in Alexandria.

Hamdeen Sabahi, a leading opposition figure, told AFP on Sunday that Egypt’s army must intervene if Morsi refuses to step down of his own volition.

“The armed forces must act, because they have always been on the side of the people,” which “has expressed its will”, said Sabahi, who came third in the 2012 presidential election, running as a left-wing nationalist.

The opposition National Salvation Front said protesters will remain in the streets until the fall of the regime. Meanwhile, the Muslim Brotherhood’s Freedom and Justice Party vowed that its activists will also remain in sit-ins to defend Morsi until the opposition rallies end.

Egypt “appeared deeply divided on Sunday with the escalation of violence,” said Al Arabiya. A presidential spokesman admitted that Morsi had made mistakes and called for dialogue.