Security forces and armed men clashed with supporters of Egypt's ousted president early on 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 Mr Morsi on July 3 and one of the deadliest in two-and-a-half 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 Mr 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 on to a nearby main boulevard, only to be confronted by police and armed civilians - reportedly residents of nearby neighbourhoods. Police initially fired tear gas but in ensuing clashes the protesters came under gunfire.
Officials from Mr 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.
US 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".
UN 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 Mr 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 on 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, Mr 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 Mr Morsi. Such offices monitored groups such as the Brotherhood, which had been outlawed for decades.
Brotherhood spokesman Ahmed Aref said that "exposes" that the regime of Mr Morsi's predecessor, Hosni Mubarak, is still alive and seeking to reverse the 2011 uprising that toppled him and led to Mr 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, one 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," Mr Ibrahim said.
Interim vice president Mohamed ElBaradei, a longtime pro-democracy campaigner who backed the military's ouster of Mr 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".