The United States urged Arab ally Egypt to pull "back from the brink" after security forces killed dozens of supporters of deposed Islamist President Mohamed Mursi and opened a dangerous new phase in the army's confrontation with his Muslim Brotherhood.

Thousands of Brotherhood supporters were hunkered down in a vigil at a Cairo mosque on Sunday, vowing to stand their ground despite a threat by the authorities to disperse them "soon".

Saturday's bloodshed, following huge rival rallies, plunged the Arab world's most populous country deeper into turmoil following two turbulent years of transition to democracy with the fall of veteran autocrat Hosni Mubarak in 2011.

Egypt's Health Ministry said 65 people had died. The Brotherhood said another 61 were on life support after what it described as a ferocious dawn assault by men in helmets and black police fatigues. The ambulance service put the death toll at 72.

Bodies wrapped in white sheets were laid on the floor of a Brotherhood morgue, their names scrawled on the shrouds.

Washington, treading a fine line with an important Middle East ally and recipient of over $1 billion in military aid, urged the Egyptian security forces to respect the right to peaceful protest.

U.S. Defence Secretary Chuck Hagel spoke by telephone with Egyptian army chief General Abdel Fattah al-Sisi, who led the July 3 military overthrow of Mursi and whose face has appeared on posters across the teeming capital, Cairo.

U.S. Secretary of State John Kerry spoke to two senior members of Egypt's army-installed interim cabinet, expressing his "deep concern."

"This is a pivotal moment for Egypt," he said in a statement. "The United States ... calls on all of Egypt's leaders across the political spectrum to act immediately to help their country take a step back from the brink."

Saturday's violence, and the threat of more, has deepened alarm in the West over events in the country of 84 million people, a vital bridge between the Middle East and North Africa.

Over 200 people have died in violence since Sisi deposed Mursi on the back of huge popular protests against his rule, ending a one-year experiment in government by the Muslim Brotherhood after decades spent in the shadows under successive Egyptian strongmen.

Pledge to stay

In the early hours of Sunday, the state-run Al-Ahram news website reported fresh confrontation in the western Helwan district of Cairo between what it described as marching Brotherhood supporters and angry residents.

The report said several cars were destroyed and gunshots heard, but there was no information on casualties.

Saturday's killings followed a day of rival mass rallies, triggered by a call from Sisi for a popular mandate to confront "violence and terrorism."

Denying police culpability, Interior Minister Mohamed Ibrahim said the vigil outside the Rabaa al-Adawia mosque in northern Cairo would "God willing, soon � be dealt with."

A public prosecutor is reviewing complaints from local residents unhappy with the huge encampment on their doorstep.

Ibrahim said angry residents had clashed with Brotherhood protesters in the early hours of Saturday, and police intervened with teargas.

Brotherhood activists said they would not be cowed and warned of worse bloodshed if the security forces did not back down. Thousands were packed into the area as night fell.

"We will stay here until we die, one by one," said Ahmed Ali, 24, as he helped treat casualties at a makeshift field hospital on Saturday.

Brotherhood spokesman Gehad El-Haddad said they would remain until their demands are met and Egypt's first freely elected president is reinstated. He accused Sisi of issuing a "clear, pre-determined order to kill."

Mursi has been held in army detention at an undisclosed location since he was deposed. Ibrahim said he would likely be transferred shortly to the same Cairo prison where Mubarak is now held, after authorities launched an investigation of him on charges including murder stemming from his 2011 escape from jail during Egypt's Arab Spring uprising.

The European Union and major European powers condemned Saturday's bloodshed, the second mass killing since Mursi's ouster. On July 8, more than 50 Brotherhood supporters died when security forces opened fire on them outside a Cairo barracks.

The events have led U.S. President Barack Obama last week to delay delivery of four F-16 fighter jets, part of some $1.5 billion a year in mainly military aid from Washington to Cairo, though U.S. officials have indicated there will be no cut-off in support to the pivotal ally.