Several witnesses from outside the protest said the gunfire started when troops appeared to move on the camp.
University student Mirna el-Helbawi told The Associated Press that she watched from her 14th floor apartment overlooking the scene, after she heard protesters banging on metal barricades, a common battle cry. El-Helbawi, 21, said she saw troops and police approaching the protesters, who were lined up on the street behind a make-shift wall. The troops fired tear gas, the protesters responded with rocks, she said.
Soon after, she heard the first gunshots and saw the troops initially retreat backward — which she said led her to believe the shots came from the protester side. She saw Morsi supporters firing from rooftops, while the troops were also shooting.
By the end, at least 51 protesters were killed and 435 wounded, most from live ammunition and birdshot, emergency services chief Mohammed Sultan, according to the state news agency.
Reeling from scenes of bloodied protesters in hospitals and clinics, many with gaping wounds, some of the country's politicians tried to push new plans for some sort of reconciliation in the deeply polarized nation.
Sheik Ahmed el-Tayeb, the grand imam of Al-Azhar, the most prominent Sunni Muslim institution, demanded that a reconciliation panel with full powers immediately start work and that those detained in recent days be released. Five prominent Brotherhood figures have been jailed since Morsi's fall, and Morsi himself is held in detention in an unknown location.
El-Tayeb's announcement he was going into seclusion was a symbolic but dramatic stance — a figure seen as a moral compass by many Egyptians expressing his disgust with all sides in the events. Egypt's Coptic popes have at times gone into seclusion to protest acts against the Christian community, but the sheik of Al-Azhar has never done so.
Struggling whether to fully bolt from the new leadership, the ultraconservative Al-Nour Party denounced what it called incitement against fellow Islamists. Speaking to Al-Jazeera TV, the party's chief Younes Makhyoun raised the possibility of calling a referendum on Morsi as a compromise measure.
There were multiple calls for an independent investigation into the bloodshed as a way to establish the truth and move forward.
The military-backed interim president, Adly Mansour, ordered a judicial inquiry into the killings. Significantly, the statement from his office echoed the military's version of events, saying the killings followed an attempt to storm the Republican Guard's headquarters.
The escalating chaos could further complicate Egypt's relations with Washington and other Western allies, which had supported Morsi as the country's first freely elected leader and now are reassessing policies toward the military-backed group that forced him out.
Still, the White House said Monday that cutting off the more than $1 billion in annual aid to Egypt was not in the U.S.'s best interests, though it was reviewing whether the military's moves constitute a coup — which would force such a measure under U.S. law.
But Egypt's new leadership appeared to be pushing ahead with the "road map" the military set up for the post-Morsi political system. Negotiations have been ongoing over appointing a prime minister, who will hold the main powers in governing the country. Talks have been stalled by Al-Nour Party vetos of candidates from liberal and secular factions — but if the party drops out, those factions may push through a candidate.
The leadership is expected to issue a declaration soon setting a timetable to create a new panel to amend or rewrite the Islamist-drafted constitution passed during Morsi's presidency — and to hold new presidential elections, possibly as early as six months from now. Al-Nour had pushed for only amendments.
At the same time, the military was pushing hard to isolate Islamists from public support, depicting their protests as rife with gunmen and weapons.
Ali said the sit-in outside the Guard headquarters had "abandoned peacefulness." Ali also pointed to other incidents of Islamist violence after Morsi's toppling on Wednesday, including coordinated, deadly attacks by extremists on military installations in the Sinai Peninsula.