CAIRO (AFP) - Egyptian security forces shot dead 16 Islamist activists early Monday during a protest in Cairo calling for ousted president Mohamed Morsi to be reinstated, his Muslim Brotherhood said.

"Sixteen people were killed and a hundred others injured, many of them in serious condition," the group's spokesman Ahmed Aref told AFP. Police barricades prevented journalists from accessing the area.

The Brotherhood said shots were fired at supporters of deposed President Mr Mursi near the military building where he is being held. The Egyptian military said "a terrorist group" had tried to storm the building. One army officer had been killed and 40 wounded, the military said.

However, Murad Ali of the Brotherhood's Freedom and Justice Party told Reuters 34 Mursi supporters had been killed. He said shooting broke out in the early morning while Islamists staged a sit-in outside the Republican Guard barracks.

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-Mursi sit-in. Ambulances were shown driving to and from the clinic.

The military overthrew Mr Mursi last Wednesday after mass nationwide demonstrations led by youth activists demanding his resignation. The Brotherhood denounced the intervention as a coup and vowed peaceful resistance against the "usurper authorities".

The ultra-conservative Islamist Nour party, which backed the military action, said it had withdrawn from negotiations to form a new interim government in protest at what it called the"massacre of the Republican Guard".

"We've announced our withdrawal from all tracks of negotiations as a first response," Nader Bakar, spokesman for Egypt's second biggest Islamist party, said on Facebook.