The crackdown followed six weeks of attempts by Western diplomats to broker a political resolution that might persuade the Islamists to abandon their protests and rejoin a renewed democratic process despite the military’s removal of Mr. Morsi, Egypt’s first freely elected president. But the brutality of the attack seemed to extinguish any such hopes.

The Health Ministry said that 235 civilians had been killed and more than a thousand others had been wounded across Egypt. But the rate of dead and seriously injured people moving through the field hospitals at the sit-ins seemed to promise the true numbers would be much higher.

The assault prompted the resignation of the interim vice president, Mohamed ElBaradei, a Nobel Prize-winning former diplomat who had lent his reputation to selling the West on the democratic goals of the military takeover.

“We have reached a state of harder polarization and more dangerous division, with the social fabric in danger of tearing, because violence only begets violence,” Mr. ElBaradei wrote in a public letter to the president. “The beneficiaries of what happened today are the preachers of violence and terrorism, the most extremist groups,” he said, “and you will remember what I am telling you.”

Image The events in Cairo set off a violent backlash across Egypt. Credit... The New York Times

The violence was almost universally criticized by Western governments. A spokesman for President Obama said the United States was continuing to review the $1.5 billion in aid it gives Egypt annually, most of which goes to the military. The spokesman, Josh Earnest, said the violence “runs directly counter to pledges from the interim government to pursue reconciliation” with the Islamists.

He said the United States condemned the renewal of the emergency law and urged respect for basic rights like the freedom of assembly and peaceful demonstrations. But he stopped short of writing off the interim government, saying the United States would continue to remind Egypt’s leaders of their promises and urge them “to get back on track.”