CAIRO — The mass shooting of Islamist protesters by security forces on Monday at a sit-in for Mohamed Morsi, the ousted president, injected new outrage into the standoff over his removal by Egypt’s top generals, darkening hopes that they might reconcile the polarizing forces that have torn at the fabric of the country.

It was by far the deadliest day of violence since the revolt that overthrew former President Hosni Mubarak in early 2011. Within a few hours around dawn, advancing soldiers and police officers killed at least 51 civilians and wounded more than 400, almost all hit by gunfire, health officials said.

Army and police spokesmen said that one soldier and two policemen had also been killed. But according to witnesses and video footage, one of the policemen appeared to have been shot by soldiers, and the military provided little evidence to back its claim that the fighting had been instigated by the Islamists.

The scale and nature of the killings drove a deeper wedge between Mr. Morsi’s Islamist backers and their opponents, and diminished the chances that his allies in the Muslim Brotherhood might soon be coaxed back into a political process that they deem illegitimate after the military overthrew the elected president.