The United States has urged Egypt to pull "back from the brink" after security forces killed dozens of supporters of deposed president Mohamed Morsi.
US Secretary of State John Kerry spoke to two senior members of Egypt's army-installed interim cabinet - Vice President Mohamed ElBaradei and Foreign Minister Nabil Fahmy - expressing his deep concern on Sunday.
"This is a pivotal moment for Egypt," he said in a statement.
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"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."
His comments came after more than 70 people were killed as security forces and Morsi supporters clashed in Cairo's Nasr City in the early hours of Saturday.
Witnesses accused security forces of using live fire, but the interior ministry said only tear gas was fired at demonstrators.
Morsi's camp said more than 100 people were killed, while the health ministry said 72 people were killed and 292 others injured.
Supporters of Morsi's Muslim Brotherhood have staged a sit-in in Nasr City since the army toppled and detained the former president on July 3.
Egypt's interim interior minister has pledged that protests calling for the reinstatement of Morsi will be dispersed "soon".
Speaking at a news conference on Saturday, Mohamed Ibrahim said security forces would act "in a legal fashion" to disperse the demonstrations "as soon as possible".
He said security forces would act to ensure "the minimum losses possible".
"We hope that [the protesters] come to their sense and that they put an end to these protests in order to prevent bloodshed," he added.
As the latest violence prompted international criticism, Human Rights Watch condemned the deaths of the protesters, accusing authorities of a "criminal disregard for people's lives".
The New York-based group said many of those killed in Cairo were shot in the head or chest, and that medical staff interviewed by their researchers "judged some of the deaths to be targeted because of the position of the shots".
The deaths suggested "a shocking willingness by the police and by certain politicians to ratchet up violence against pro-Morsi protesters," HRW's deputy Middle East and North Africa director Nadim Houry said in a statement.
The group urged Egypt's interim government and military leaders to "immediately order an end to the use of live gunfire except where strictly necessary to protect life".
In another incident, one person was killed and 28 others injured in Port Said in overnight clashes, state news agency MENA reported.