CAIRO—Egypt's military regime, aided by snipers and bulldozers, swept the streets of Islamist protesters Wednesday—setting off a day of violence that left at least 525 people dead, the government fractured and ties with its international partners in tatters.

Cairo's streets were calm Thursday morning following a curfew overnight, though funerals for the dead and a march planned by the Muslim Brotherhood are likely to inflame tensions later in the day.

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