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Sarah Lynch, Special for USA TODAY

CAIRO - Opposing political camps appeared no closer to resolving a deadly political impasse in Egypt on Sunday as the death toll in weekend clashes climbed and prospects for more violence billowed.

At least 80 people were killed in clashes over the weekend, Egypt's Health Ministry said. Seventy-two people died in Cairo alone from violence that erupted Saturday morning between security forces and supporters of ousted President Mohamed Morsi, making it one of the deadliest bouts of unrest the country has seen since the 2011 uprising against Hosni Mubarak.

At least eight others were killed in clashes that started Friday in the northern city of Alexandria, the Health Ministry said. More than 700 people in total were injured in violence Friday and Saturday.

Kerry on Saturday called on Egyptian leaders to release detained political leaders so that a meaningful political dialogue can proceed.

"This is a pivotal moment for Egypt," Kerry said after consulting with European Union foreign policy chief Catherine Ashton and speaking to Egypt's interim vice president and foreign minister. "The final verdict" on the revolution that began more than two years ago "is not yet decided," Kerry said. "But it will be forever impacted by what happens right now."

Competing narratives emerged over who was to blame for Saturday's deaths.

The interior ministry denied security forces opened fire, claiming they only used tear gas as Morsi supporters tried to extend a sit-in, blocked a major road and clashed with local residents. The resulting violence injured 14 police officers including two with gunshot wounds to the head, a ministry statement said.

The Muslim Brotherhood, however, which propelled Morsi to power, said police, snipers and thugs attacked demonstrators in the early hours of the morning.

"They are killing us," said Waiel Yahya, a member of the Muslims Brotherhood's political wing, as he stood in a field hospital that housed some of the dead.

Many of those killed were shot in the chest or head during the clashes, which went on for at least six hours, Human Rights Watch said.

"It is almost impossible to imagine that so many killings would take place without an intention to kill, or at least a criminal disregard for people's lives," said Nadim Houry, deputy Middle East and North Africa director at Human Rights Watch, in a report.

Interior Minister Mohamed Ibrahim said Saturday in a televised news conference that pro-Morsi protesters who remain in the streets will be dispersed "soon."

Amid the unrest, Cairo's U.S. Embassy closed to the public on Sunday, the start of the working week in Egypt.

The violence followed rival protests that jammed streets across the country Friday. Morsi supporters demanded the ousted leader's reinstatement and protested against Gen. Abdel Fattah Al-Sisi.

"We came here to defend our votes in elections," said Hamed Al Mekabty, 31, as he marched in a boisterous Cairo rally. "We are all against the military coup and came to say to Sisi: 'Go away, we don't want you.' "

Morsi's opponents flowed into Tahrir Square and crowded outside the presidential palace. They showed their support for the military and Al-Sisi, who had called on Egyptians to rally to give him and security forces a mandate to tackle "possible violence and terrorism."

"I think Sisi will make good decisions about the future of Egypt," said Heba Abdel Salem, who rallied in Tahrir Square. "I hope we will see the end of the Brotherhood."

Hours later, the Cairo violence began and dozens of Morsi supporters were killed near a sit-in they have held for weeks outside Rabaa Al-Adawiya Mosque in the capital's Nasr City.

"The use of deadly fire on such a scale so soon after the interim president announced the need to impose order by force suggests a shocking willingness by the police and by certain politicians to ratchet up violence against pro-Morsi protesters," Houry said in the human rights report.

The atmosphere in the country has grown increasingly charged since protests erupted in late June. Millions of Egyptians took to the streets to demand that Morsi resign and call for an early presidential vote. Days later, on July 3, Al-Sisi ushered in a transition plan that unseated Morsi, the nation's first freely elected president, and paved the way for new parliamentary and presidential elections under an interim government.

An Egyptian court on Friday ordered that Morsi be detained for 15 days pending investigation. The ousted leader is charged with conspiring with the Palestinian group Hamas in a violent prison break, among other accusations.

Interior Minister Mohamed Ibrahim said Saturday that Morsi will likely be transferred to the same prison that is holding Mubarak.

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