(Image Credit: Manu Brabo/AP Photo)

Supporters of former Egyptian President Mohamed Morsi remain defiant after the deadliest weekend since Morsi was ousted July 3. Raising the prospect of another bloody confrontation, the Muslim Brotherhood has called for mass rallies this week after the military-backed civilian government threatened to clear the protesters off the streets.

Thousands of Morsi supporters marched early this morning from Cairo's Nasr City, where they have been camped out for weeks in protest, toward Cairo's military intelligence headquarters despite an explicit warning from the army to stay away.

Chanting "our blood and souls we sacrifice for Morsi," the protesters remained peaceful but the message was clear: the Brotherhood will not go away quietly.

"We're not backing down," Muslim Brotherhood spokesman Gehad El-Haddad told ABC News.

"It's time for the world to act," he said, adding that the sit-in may continue for months.

Morsi supporters have called for a march tonight toward government security buildings in advance of a "million man march" planned for Tuesday.

The tension in Egypt's capital reached a fever pitch this weekend after dozens of Morsi supporters were reportedly gunned down Saturday morning. The Health Ministry says at least 80 people were killed and hundreds injured in a predawn raid. The Muslim Brotherhood puts the death toll much higher.

There were conflicting accounts about who started the violence and both sides blame the other. Body bags streamed out of a field hospital in Cairo's Nasr City for hours Saturday morning to make room for hundreds of wounded protesters arriving.

The Brotherhood says Egyptian security forces blatantly attacked unarmed, peaceful protesters before Friday morning prayers. But the Ministry of Interior took no blame, saying the security forces would never shoot an Egyptian civilian.

The Ministry's official account reads in stark contrast to an independent report by Human Rights Watch, which found many casualties shot in the head or chest from a high angle, possibly nearby rooftops.

There's a sense that Saturday's violence was merely the warm-up round and an even tougher military crackdown might be on its way. Strategically, however, the Egyptian military doesn't have a great next move. Any kind of political compromise with the Muslim Brotherhood is looking increasingly unlikely, and it's hard to see how the tens of thousands of people camped out in eastern Cairo could be cleared without serious bloodshed.

Nearly a month after Morsi was deposed in a military takeover, the Brotherhood sees little incentive to get off the streets and join the political discussion.

Haddad told ABC News that there is "nothing at all" the government could do to placate the Brotherhood.

The military-backed government is holding much of the party's leadership incommunicado, including Morsi, who has not been seen publicly since July 3. Late Sunday night, two more senior leaders, Abul Ela-Madi and Essam Sultan were arrested in eastern Cairo and taken to the capital's Tora prison where former president Hosni Mubarak is currently serving out a life sentence.

Haddad expects the crackdown to intensify. "The government is thinking like a military machine, not like civilian leaders," he said.