CAIRO - Egyptian security forces shot dead dozens of supporters of ousted Islamist President Mohamed Morsi on Saturday, witnesses said, days after the army chief called for a popular mandate to wipe out "violence and terrorism".

The bloodshed, near the military parade ground where President Anwar Sadat was assassinated in 1981, plunged the Arab world's most populous country deeper into turmoil following two turbulent years of transition to democracy since veteran autocrat Hosni Mubarak was swept from power.

Supporters of deposed Egyptian President Mohamed Morsi shout slogans during a protest at the Rabaa Adawiya square, where they are camping, in Cairo, July 27, 2013. [Photo/Agencies]

Morsi's Muslim Brotherhood, braced for a military crackdown, said men in helmets and black police fatigues had fired on crowds gathered before dawn on the fringes of a round-the-clock sit-in near a mosque in northeast Cairo.

Activists rushed blood-spattered casualties into a makeshift hospital. Some were carried in on planks or blankets. One ashen teenager was laid out on the floor, a bullet hole in his head.

The United States, treading a fine line with an important Arab ally and recipient of over $1 billion a year in US military aid, urged respect for the right to peaceful protest, and warned that Egypt was at a "pivotal moment".

Brotherhood spokesman Ahmed Aref said 66 people had been killed and another 61 were "brain dead" on life support machines. More than 4,000 were treated for the effects of tear gas and gunshot or birdshot wounds, he told reporters.

"Innocent blood was spilled," Aref said. "We have gone back 10 years."

The Health Ministry reported a total of 65 dead, while the head of the ambulance service, Mohamed Sultanm, said later that 72 had died.

"They are not shooting to wound, they are shooting to kill," another Brotherhood spokesman, Gehad El-Haddad, told Reuters early on Saturday. "The bullet wounds are in the head and chest."

Egypt's Interior Minister Mohamed Ibrahim denied that police had opened fire, saying local residents living close to the Rabaa al-Adawia mosque vigil had clashed with protesters in the early hours after they had blocked off a major bridge road. He said police had used teargas to try to break up the fighting.

Well over 200 people have been killed in violence since the army toppled Morsi on July 3, following huge protests against his year in power. The army denies accusations it staged a coup, saying it intervened to prevent national chaos.