At least 51 people were killed on Monday when demonstrators enraged by the military overthrow of Egypt's elected Islamist president said the army opened fire during morning prayers outside the Cairo barracks where Mohamed Morsi is believed held.
But the military said "a terrorist group" tried to storm the Republican Guard compound and one army officer had been killed and 40 wounded. Soldiers returned fire when they were attacked by armed assailants, according to a military source.
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In the deadliest incident since Morsi's removal, emergency services said more than 430 were wounded.
Meanwhile, Sky Arabia reported that the majority of army forces withdrew from the scene of the clashes on Monday morning in an attempt to calm tensions.
Morsi's Muslim Brotherhood urged people to rise up against the army, which they accuse of a military coup to topple the leader, threatening an escalation in Egypt's political crisis.
Cairo in flames early Monday morning (Photo: AFP)
Murad Ali of the Brotherhood's Freedom and Justice Party said said shooting broke out in the early morning while Islamists staged a sit-in outside the Republican Guard barracks.
Body of one of the victims (Photo: Reuters)
Al Jazeera's Egypt news channel broadcast footage of what appeared to be five men killed in the violence, and medics applying cardiopulmonary resuscitation to an unconscious man at a makeshift clinic at a nearby pro-Morsi sit-in.
Casualties in Cairo
A spokesman for Morsi's Muslim Brotherhood said its members were unarmed and were fired upon while praying outside the building.
This is the largest death toll from a single incident in Egypt since the ouster of Hosni Mubarak in 2011.
The Islamist Al-Nour party, which backed Morsi's ouster, called the incident a "massacre" and announced it will not take part in any political negotiation.
Wounded evacuated (Photo: Al-Jazeera)
Egypt's state TV aired images from Monday's clashes showing some of the Islamist protesters firing at army forces and hurling firebombs and stones at the Presidential Guard facility.
An army spokesman said that the clashes erupted after armed members of the Muslim Brotherhood began to fire at the soldiers using shot guns.
Egyptian TV aired photos of the weapons seized from the protesters including knives, chains and hand grenades.
It was also reported that unknown assailants kidnapped two soldiers in west Cairo. They were later released according to reports.
On Sunday night, opponents of Morsi packed Tahrir Square in the tens of thousands to show the world his ouster was not a military coup but the reflection of the people's will.
Army in Cairo (Photo: Getty Images)
Air force jets circling sky (Photo: EPA)
The Tahrir gathering was staged as a counter-demonstration two days after Islamist rallies exploded into deadly violence, the protest raised the stakes as the country's interim leaders struggled to put together a new government.
As the crowds grew, wave after wave of military aircraft skimmed over the capital, with one formation leaving behind long trails of smoke in black, white and red – the colors of the Egyptian flag.
"We are on the street to show the world that it was a popular revolution and not a coup that overthrew" Morsi on Wednesday, said a beaming teacher who gave her name as Magda.
Reuters, AP and Roi Kais contributed to this report
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