The U.N.'s human rights chief, Navi Pillay, also condemned the violence and called for a "credible, independent investigation" into the killings.
"I fear for the future of Egypt if the military and other security forces, as well as some demonstrators, continue to take such a confrontational and aggressive approach. Supporters of the Muslim Brotherhood have the right to protest peacefully like anyone else," Pillay said.
The violence continued Sunday, when deadly clashes during funerals for two of the slain Morsi supporters left two men dead and scores injured in two cities north of Cairo, Port Said and Kafr el-Zayat.
Interior Minister Mohammed Ibrahim warned security forces would deal decisively with any attempts to destabilize the country. Ibrahim accused the pro-Morsi side of provoking bloodshed to win sympathy and suggested authorities would move against the two pro-Morsi protest camps outside the Rabaah al-Adawiya mosque in eastern Cairo and in Nahda Square, near the main campus of Cairo University.
"I assure the glorious people of Egypt that the police are determined and capable to maintain security and safety to their nation with the support of the sincere sons of the country," Ibrahim said during a graduation ceremony at the national police academy. "We will very strongly and decisively deal with anyone who attempts to undermine stability."
He depicted the two encampments as a danger to the public, pointing to nine bodies found nearby in recent days. Some had been tortured to death, police said, apparently by sit-in participants who believed they were spies. "Soon we will deal with both sit-ins," he said.
Setting the stage for more confrontation, the military-installed interim president gave the prime minister the power to grant the military the right to arrest civilians in what government officials suggested was a prelude to a major crackdown on Morsi's supporters or Islamic militants who have stepped up attacks against security forces in the Sinai Peninsula.
At least 20 members of the security forces have been killed in Sinai by suspected militants and nearly 250 in the rest of the country, including the 83 killed in Cairo on Saturday. Late Sunday, a member of the army's elite Saaqa commandos was killed in a mortar and rocket attack by suspected militants that wounded four others in northern Sinai, a stronghold of radical Islamists.
"The more bloodshed there, the more it is impossible to reach a compromise or middle ground," said Kamal Habib, a prominent scholar in Islamic movements and a former Islamist himself.
The two sides, he said, were gearing toward more confrontations.
A senior aide to interim President Adly Masnour, meanwhile, sought to prepare the public for possible action to dismantle the sit-in camps, telling reporters that Morsi's supporters were armed, terrorizing residents in the area.
"It has now become inevitable for the state to take measures necessary to protect society," he said.
The nation's highest security body — the National Defense Council — issued a statement saying the pro-Morsi sit-in camps violated Egypt's national security and warning that "decisive and firm" action would be taken. It also urged the protesters to renounce violence and stop "violence and terrorism and verbal and physical assaults on citizens."
The council, chaired by the interim president and including the prime minister, defense and foreign ministers, said it deeply regretted the loss of life, but did not blame any party for it.
Saturday's clashes — the deadliest since more than 50 Morsi supporters were killed by troops on July 8 — took place before dawn when police and armed men in civilian clothes opened fire on supporters of the former president as they tried to expand their sit-in camp outside Rabaah al-Adawiya by moving onto a nearby main boulevard.
Civilians, sometimes with weapons, frequently join police in Cairo demonstrations. In some cases, they appear to be plainclothes police, in others residents who back the security forces.
Videos posted Sunday on social networking sites showed the Morsi supporters approaching a police line backed by armored vehicles at the entrance of the ramp to a key bridge that runs across the heart of the city. They also showed police and men in civilian clothes pointing their rifles at the protesters, many of whom wore industrial helmets and homemade body armor and stood behind makeshift barricades.
Mohamed Wasfi, a children's book publicist who videotaped the clashes from his apartment balcony, said the protesters attempted to spill oil on the street to stop cars from approaching the bridge, a tactic used by Morsi supporters last week on an overpass that leads to Cairo's international airport. Shortly afterward, another group of protesters approached the police line and tore down metal barricades, prompting police to fire tear gas, he told The Associated Press.