CAIRO -- Egypt’s fragile political condition sank toward critical Monday after the military opened fire and killed dozens of Islamists who were demanding the return to office of deposed President Mohammed Morsi. It was the worst political violence in the country since the demonstrations two and a half years ago that led to the resignation of Hosni Mubarak.

Adly Mansour, the Constitutional Supreme Court judge whom the military named president last week to replace Morsi, tried to defuse tension by announcing that a new constitution would be submitted for approval to the people within four months and elections to select a new Parliament would take place by February. A presidential vote would follow a week after Parliament convened.

But the promise of a quick return to elected leadership seemed unlikely to assuage the bitterness of Morsi supporters who were targeted by live gunfire outside the headquarters of the Republican Guard in eastern Cairo before dawn, killing an estimated 51.

The Brotherhood’s Freedom and Justice political party summoned its supporters to demonstrate Tuesday, and Brotherhood members vowed to avenge the deaths, a portent of what’s likely to be protracted conflict between the millions who backed the military’s removal of Morsi and those who consider it an illegal coup that removed a democratically elected leader. The Brotherhood said it wouldn’t stop its demonstrations or consider reconciliation until Morsi was back in office.

The military, meanwhile, defended its use of live gunfire, claiming that hundreds of Morsi supporters had opened fire first as they tried to force their way into the Republican Guard facilities, where many think Morsi is being held. It said two police officers and a soldier had died in the attack. At least 435 people were injured in the melee.

The top cleric for the Sunni Islamic world warned of potential civil war. Sheikh Ahmed al Tayeb, the leader of Cairo’s al Azhar mosque, issued yet another call for peace but couldn’t hide his frustration. He announced that he’d go into seclusion until the crisis ended.

Before Monday’s clashes, most of the violence since Morsi was removed from office last Wednesday had been between partisans of the two sides, with 40 killed and 1,000 injured.

But Monday’s events stunned even the most jaded here. For the first time, Egypt’s revered military had opened fire on its citizens and caused many casualties, and many feared there would be no preventing further bloodshed. The Nour Party, the only Islamist body represented in the transitional government, resigned from the government in protest.

Reaction in the streets of Cairo underscored the high emotions that have characterized the conflict.

“Kill them all. Why can’t they accept that we don’t want them!” screamed a fruit vendor named Mohammed, referring to Morsi’s supporters as he watched television news accounts of what took place.

The scene of the shooting was horrific. Rivers of blood ran through the street, which was littered with the signs of panic: eyeglasses, shoes and clothing discarded in the rush to find safety. The area smelled of blood.

The bodies of the dead overwhelmed morgues, and hospitals were hard-pressed to deal with the hundreds of wounded.

The army and the Muslim Brotherhood offered different versions of events and sought to prove that they weren’t responsible for the bloodshed.