Brotherhood spokesman Ahmed Aref said that "exposes" that the regime of Morsi's predecessor, Hosni Mubarak, is still alive and seeking to reverse the 2011 uprising that toppled him and led to Morsi's election.
Despite the heavy death toll, the interior minister suggested authorities could take the more explosive step of moving against the two main pro-Morsi protest camps in Cairo: weeks-old sit-ins, on outside the Rabaah al-Adawiya in eastern Cairo and another in Nahda Square in Cairo's sister city of Giza.
He depicted the encampments as a danger to the public, pointing to a string of nine bodies police have said were found nearby in recent days. Some had been tortured to death, police have said, apparently by members of the sit-ins who believed they were spies.
"Soon we will deal with both sit-ins," Ibrahim said.
Interim Vice President Mohamed ElBaradei, a longtime pro-democracy campaigner who backed the military's ouster of Morsi, raised one of the few notes of criticism of Saturday's bloodshed.
"I highly condemn the excessive use of force and the fall of victims," he wrote in a tweet, though he did not directly place blame for the use of force. He added that he is "working very hard and in all directions to end this confrontation in a peaceful manner."
But the image of the Islamists as dangerous and not the peaceful protesters they contend they are has had a strong resonance. Over past weeks, there have been cases of armed Islamist Morsi backers attacking opponents — though the reverse also has occurred. Before Saturday, some 180 people had been killed in clashes nationwide.
Walid el-Masry, one of founders of the youth activist Tamarod movement that led the original wave of protests against Morsi, said he believed the Brotherhood "pushed for (the) clashes. ... The Muslim Brotherhood is trying to grab the international attention and have the victim attitude."
The liberal umbrella group National Salvation Front, which ElBaradei once led, also said it "puts strong blame on the Brotherhood," pointing to hard-line rhetoric in speeches at pro-Morsi rallies calling for "jihad" and "martyrdom."
The clashes began after a crowd of Morsi supporters late Friday moved out of their Rabaah al-Adawiyah encampment and installed themselves on a nearby major thoroughfare, blocking it. They began to set up tents there, planning to stay there at least three days, said Mahmoud Zaqzouq, a Brotherhood spokesman. A march also attempted to cut off a major overpass that runs through Cairo.
Police moved in and fired tear gas to break up the crowds at around 2 a.m., and protesters responded with volleys of stones.
Gunshots also rang out, seemingly from both sides, said one witness, Mosa'ab Elshamy, a freelance photographer, though he could not tell who started firing.
Armed residents of the area also joined the police side, and there were also plainclothes police carrying handguns, he said.
"They aimed at killing the people. They aimed the head and the neck," said Ahmed Abdullah, a doctor at a field clinic set up at Rabaah al-Adawiya, as he wiped away tears.
At the Rabaah al-Adawiya clinic, men shouted "God is great," and women wailed as bodies were loaded into ambulances to be taken for examination at hospitals. Bodies of more than a dozen men lay on the blood-splattered floor with white sheets over them.
Ragab Nayel Ali, one of the pro-Morsi protesters, said security forces fired first with tear gas and birdshot. "Protesters replied by hurling rocks and started building walls," Ali said.
Health Ministry spokesman Khaled el-Khateeb said that at least 65 people were killed and 270 wounded. Nine more were killed in clashes in the Mediterranean city of Alexandria since Friday, he said.
Aref of the Brotherhood told reporters that another 61 were "clinically dead." He did not further explain their condition.
The Interior Ministry said 14 policemen were wounded, two with gunshot wounds to the head.
Clashes in Alexandria erupted on Friday and extended into the night as more than 100 Morsi supporters took refuge in a central mosque, holding 17 of their rivals hostage as they tried to fend off a security siege of the building. A security official said the hostages were freed and many of those inside the mosque arrested. The official spoke on condition of anonymity because he was not authorized to speak to journalists.
A leading Brotherhood figure, Mohammed el-Beltagy, accused army chief el-Sissi of seeking violence by calling Friday's pro-military rallies.
"This is the mandate el-Sissi took last night — to commit massacres and bloodshed against peaceful protesters denouncing the military coup," el-Beltagy said in a statement on his Facebook page.
Associated Press writers Sarah El Deeb and Mariam Rizk contributed to this report.
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