"Bullets whizzing"

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."

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

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.

Egyptian-born Youssef al-Qaradawi, one of the most prominent Sunni clerics in the Middle East, urged Egyptians to join the Brotherhood vigil, according to a statement on his website.

"People must come out of their homes to stand in the face of the criminal murderers with all bravery," he said.

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