CAIRO (AP) - Security forces and armed men clashed with supporters of Egypt's ousted president early Saturday, killing at least 65 people in mayhem that underscored an increasingly heavy hand against protests demanding Mohammed Morsi's return to office. In chaotic scenes, pools of blood stained the floor and bodies were lined up under white sheets in a makeshift hospital near the site of the battles in eastern Cairo. Doctors struggled to cope with the flood of dozens of wounded, many with gunshots to the head or chest. It was the deadliest single outbreak of violence since the military ousted Morsi on July 3 and one of the deadliest in 2 ½ years of turmoil in Egypt. It was not immediately clear if all the 65 killed were all protesters or if residents who joined the fight against the march were among the dead. The Brotherhood said that 66 Morsi supporters were killed in the Cairo violence. The extent of the bloodshed pointed to a rapidly building confrontation between the country's two camps, sharply divided over the coup that removed Egypt's first freely elected president after widespread protests against his rule. Authorities talk more boldly of making a move to end weeks of protests by Morsi's largely Islamist supporters. At the same time, the Islamists are growing more assertive in challenging security forces as they try to win public backing for their cause. Saturday's clashes were sparked when pro-Morsi protesters sought to expand their main Cairo sit-in camp by moving onto a nearby main boulevard, only to be confronted by police and armed civilians - reportedly residents of nearby neighborhoods. Police initially fired tear gas but in ensuing clashes, the protesters came under gunfire. Officials from Morsi's Muslim Brotherhood and their allies decried what they called a new "massacre" against their side, only weeks after July 8 clashes with army troops in Cairo that left more than 50 Morsi supporters dead. U.S. Secretary of State John Kerry said that he spoke to Egyptian authorities, saying it is "essential" they respect the right to peaceful protest. He called on all sides to enter a "meaningful political dialogue" to "help their country take a step back from the brink." U.N. Secretary-General Ban Ki-moon also asked security forces to "act with full respect for human rights" and demonstrators to "exercise restraint." But neither side has shown much taste for reconciliation. Islamists staunchly reject the new leadership and insist the only possible solution to the crisis is to put Morsi back in office. Meanwhile, the interim leadership is pushing ahead with a fast-track transition plan to return to a democratically elected government by early next year. The military-backed authorities appear confident of public support for a tougher hand after millions turned out for nationwide rallies Friday called by army chief Gen. Abdel-Fattah el-Sissi as a mandate against "terrorism and violence." Interior Minister Mohammed Ibrahim, who is in charge of police, took an uncompromising stance in a news conference after the violence. He accused the pro-Morsi side of provoking bloodshed to win sympathy. "We didn't go to them, they came to us - so they could use what happened for political gain," he said. Ironically, Ibrahim is originally a Morsi appointee, and his then-boss praised him for a tough hand after police killed dozens of anti-Morsi protesters in the city of Port Said earlier this year. "The Ministry of Interior never has and never will fire on any Egyptian," he added, saying police only shot tear gas in Saturday's violence. The minister also said there were plans to bring back "political security" offices dissolved under Morsi. Such offices monitored groups like the Brotherhood, which had been outlawed for decades. 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. 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. 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. 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. "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. 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. Associated Press writers Sarah El Deeb and Mariam Rizk contributed to this report.