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

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

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.

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," he said. "We have gone back 10 years."

A Muslim Brotherhood website said earlier that 120 people had been killed. A Reuters reporter counted 36 bodies at one morgue. 

Hamdi Mahmoud, director of the health ministry press office, told Reuters that 65 had died, and a further nine were killed in overnight violence in Egypt's second city, Alexandria.

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.

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

Egypt's Interior Minister Mohamed Ibrahim accused the Brotherhood of exaggerating the death toll for political ends and denied that police had opened fire.

Ibrahim said 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 that 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.

Sisi's challenge

Hundreds of thousands of Egyptians had poured onto the streets on Friday in response to a call by army chief General Abdel Fattah al-Sisi for nationwide demonstrations to give him backing to confront the weeks-long wave of violence.

His appeal was seen as a challenge to the Brotherhood, which organized its own rallies on Friday calling for the return of Morsi, who has been held in an undisclosed location since his ousting and faces a raft of charges, including murder.

Ibrahim said Morsi was likely to be transferred shortly to the same Cairo prison where former leader Mubarak is now held.

Brotherhood leaders appealed for calm on Saturday, but activists at the Rabaa al-Adawia mosque vigil voiced fury.

"The people want the execution of Sisi," a cleric shouted to the crowd from a stage by the mosque. "The people want the execution of the butcher."

Interior Minister Ibrahim said the pro-Morsi sit-ins would "God willing, soon ... be dealt with" based on a decision by a public prosecutor, who is reviewing complaints from local residents unhappy with the huge encampment on their doorstep.

The head of the Nour Party, the second-biggest Islamist group after the Brotherhood, called for an immediate investigation into what it called a "massacre".

The Brotherhood is a highly organized movement with grassroots support throughout Egypt, making it hard to silence even if the army decides to mount a bigger crackdown.

European Union foreign policy chief Catherine Ashton said she "deeply deplores" Saturday's deaths and urged all sides to halt the violence. There was no immediate comment from the United States, which provides Egypt with some $1.5 billion dollars of aid a year, mainly military hardware.

Washington has delayed delivery of four F-16 fighters because of the turmoil. However, officials have indicated they do not intend to cut off aid to a country seen as a vital ally and which has a peace deal with neighboring Israel.

'Bullets whizzing'

Liberal leaders in Egypt, many of whom backed the army's move against Morsi, also voiced concern.

Writing on Twitter, Mohamed ElBaradei, vice president in the army-installed interim administration, said: "I condemn excessive use of force and the casualties. I am working with all effort in every direction to end the confrontation through peaceful means."

Witnesses said police first fired rounds of teargas at Brotherhood protesters gathered on a boulevard leading away from the Rabaa mosque, with live shots ringing out soon afterwards.

"There were snipers on the rooftops, I could hear the bullets whizzing past me," said Ahmed el Nashar, 34, a business consultant, choking back his tears.

"Man, people were just dropping."

Dr. Ibtisam Zein, overseeing the Brotherhood morgue, said most of the dead were hit in the head, some between the eyes.

The bodies were wrapped in white sheets and laid on the floor, their names scrawled on the shrouds. A cleaner busily mopped the floor, washing away pools of blood.

Haddad said the Brotherhood remained committed to pursuing peaceful protests, despite Saturday's deaths - the second mass shooting of its supporters this month by security forces, who killed 53 people on July 8.

Brotherhood activists at Rabaa said they would not be cowed and warned of worse bloodshed if the security forces did not back down. "We will stay here until we die, one by one," said Ahmed Ali, 24, helping treat casualties at the field hospital.

"We have the examples of Algeria and Syria in our minds. We don't want it to become a civil war. If we take up arms it might become one. This is a religious belief."