CAIRO - Deadly violence erupted before the sun rose over Cairo on Monday morning at a sit-in by supporters of former Egyptian President Mohammed Morsi, leaving many dead.
Egypt's health ministry said at least 42 people were killed. A Muslim Brotherhood spokesman said that the death toll included five children.
Egyptian military officials said only five supporters of the now-deposed president were killed, the Associated Press reported, but hundreds - Egypt's health ministry said 322 - are believed to be injured.
The exact course of events Monday morning remains unclear.
The military said people tried to storm a Republican Guard facility in Cairo's Nasr City, according to the Associated Press. But Muslim Brotherhood spokesman Gehad El-Haddad, who was at a sit-in with his family near the violence, said security forces fired on peaceful demonstrators.
Police and military attacked people outside the Republic Guard facility as people were praying, firing rounds of live ammunition and shotgun pellets, El-Haddad said on his Twitter account. Many shots were aimed at the feet, resulting in a lot of leg wounds, he said, and security forces fired tear gas into the crowd in an attempt to disperse protesters.
The wounded were taken to a field hospital set up in the area.
"This is a crime against humanity," said Hesham Al Ashry, an ultraconservative Islamist who was protesting but left the area before violence broke out.
Last week, military officers killed four Brotherhood demonstrators, who were unarmed, in the same location as Monday's violence.
The military has a history of using excessive force to quell demonstrations. One of the most brutal cases took place in October 2011 in an area of Cairo called Maspero. The military ran over some demonstrators with massive military vehicles at a mainly Coptic Christian rally. Twenty-seven people were killed, including a military officer.
Monday's violence heightens the conflict between military and Morsi supporters, who call the military's move to oust Morsi a "military coup." They have refused to stop protesting until Morsi is released from detention and reinstated as president.
Egypt's opposition believes the army's move to set the nation on a new transitional path - suspending the constitution, dissolving the legislature and appointing a new interim president and government -were justified and backed by popular support.
But many in the pro-Morsi political camp believe all the votes they cast over the past two-and-a-half years have been stolen. They have been staging a sit-in since last week.
"We only came here to defend our vote," said Nassser Ibrahim, a teacher, at a pro-Morsi rally Sunday afternoon not far from where Monday's violence later erupted.
The shootings come at a fragile time in Egypt's transition after Morsi was ousted from power last Wednesday and threaten to unhinge a delicate political system.
Egypt's Nour Party - a hardline Islamist group - said Monday that it was withdrawing from negotiations over who will be named to the new government in response to the "massacre."
Prior to Monday's violence, the party recently rejected the appointment of liberal figure Mohamed ElBaradei as prime minister. The group's recent withdrawal from talks threatens to further stall, or even paralyze, the new transition.
As tension between opposing political camps rose over the last week, dozens have been killed in clashes. One of the highest death tolls came after clashes broke out last Friday nationwide without much interference from security forces.
"Neither the police nor the military effectively intervened in deadly clashes between pro- and anti-Muslim Brotherhood supporters that left 36 people dead on July 5, 2013," Human Rights Watch said.
Sarah Lynch, Special for USA TODAY