At least 35 people were killed on Monday in Cairo, an Egyptian Health Ministry spokesman said,
when the Muslim Brotherhood said shots were fired at supporters of
deposed President Mohamed Morsi near the military building where he is
The bloodshed deepened Egypt's political crisis, escalating the struggle between the army, which overthrew Morsi last Wednesday after mass demonstrations demanding his resignation, and the Brotherhood, which has denounced what it called a coup.
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, a military source said.
The Brotherhood's official spokesman, Gehad El-Haddad, who is at a pro-Mursi sit-in at a mosque near the scene, said 37 Mursi supporters had been killed.
He said shooting broke out in the early morning while Islamists were praying and staging a peaceful sit-in outside the Republican Guard barracks.
"We call on all patriotic brave Egyptians 2 join us @... sitin to defend country from conspiratorial traitors of military coup," he said in a Twitter message.
As an immediate consequence, the ultra-conservative Islamist Nour party, which initially supported the military intervention, said it was withdrawing from stalled negotiations to form an interim government for the transition to fresh elections.
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.
A Reuters television producer at the scene saw first aid helpers attempting mouth-to-mouth resuscitation on a dying man. Wounded people were being ferried to the field hospital on motorbikes, given first aid treatment and taken away in ambulances.
The military overthrew Morsi on Wednesday after mass nationwide demonstrations led by youth activists demanding his resignation. The Brotherhood denounced the intervention as a coup and vowed peaceful resistance.
Military vehicles sealed off traffic in a wide area around the Rabaa Adawia mosque where Morsi supporters led by senior Brotherhood leaders have been staging protests since his ouster.
The army also closed two of the main bridges across the Nile River with armored vehicles, witnesses said.
Talks on forming a new government were already in trouble before Monday's shooting, after the Nour party rejected two liberal-minded candidates for prime minister proposed by interim head of state Adli Mansour.
Nour, Egypt's second biggest Islamist party, which is vital to give the new authorities a veneer of Islamist backing, said it had withdrawn from the negotiations in protest at what it called the "massacre at the Republican Guard (compound)".
"We've announced our withdrawal from all tracks of negotiations as a first response," party spokesman Nader Bakar said on Facebook.