The top US diplomat said he had spoken to two senior officials of Egypt's interim government - Vice President Mohamed ElBaradei and Foreign Minister Nabil Fahmy - to voice "deep concern about the bloodshed and violence in Cairo and Alexandria over the past 24 hours that has claimed the lives of scores of Egyptian demonstrators and injured more than 1,000 people."
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Egyptian security forces shot to death dozens of supporters of ousted Islamist President Mohamed Morsi, witnesses said, days after the army chief called for a popular mandate to wipe out "violence and terrorism".
"This is a pivotal moment for Egypt," Kerry said in a written 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."
He also called on the Egyptian security forces to respect Egyptians' right to protest peacefully, saying that was "a moral and legal obligation."
"At this critical juncture, it is essential that the security forces and the interim government respect the right of peaceful protest, including the ongoing sit-in demonstrations," he added.
He also urged an inclusive political process involving representatives of all political parties that would lead as soon as possible to "a freely and fairly elected government committed to pluralism and tolerance."
Violence against protestersBrotherhood 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.
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