The Muslim Brotherhood has declared a national uprising after dozens of supporters of ousted President Mohamed Mursi were shot dead.
Egyptian state TV reported that 42 people were killed and more than 300 injured outside the Cairo headquarters of the Republican Guard where Mursi is being held.
The army said a “terrorist group” had tried to storm the building.
The Brotherhood said its supporters were praying when the military opened fire.
One injured man said: “The people who were firing the most were the military. The police were also firing, but the military were firing more. We’ve been betrayed. I saw them with my own eyes.”
The bloodshed has deepened Egypt’s political crisis, between the army, which overthrew the elected Islamist President and the Brotherhood, which has denounced a military coup.
As a consequence of the shootings, the Islamist Nour party has pulled out of negotiations to form an interim government.