Riot police stood guard outside the Fateh Mosque in Cairo’s central Ramses Square, which had been transformed into a field hospital and morgue on Friday during fierce clashes in the area. Tanks guarded the mosque’s back entrance, and at about noon, witnesses said they heard gunfire.

The standoff came as Mr. Morsi’s supporters prepared for new demonstrations, raising the likelihood of further violence. Egyptian officials announced that they had arrested hundreds of Islamists over the last two days, as part of an expanding crackdown on the Muslim Brotherhood, Egypt’s most powerful Islamist movement.

The scenes outside the mosque, broadcast live on television on Saturday, deepened the sense that Egypt was descending into anarchy. On Friday, terrified protesters caught in a cross-fire jumped or fell from an overpass in a panicked effort to escape. A gunfight erupted on the doorstep of a Four Seasons hotel. Men wielding guns and machetes — some backing the Islamists, others police supporters in civilian clothes, others simply criminals — roamed the streets of the capital and other cities, and it was often impossible to tell friend from foe.