A young man next to the bodies of protesters killed on Wednesday. Many of the dead were shot in the head or chest; some appeared to be in their early teens.

CAIRO — Egyptian security officers stormed two encampments packed with supporters of the ousted president, Mohamed Morsi, on Wednesday in a scorched-earth assault that killed hundreds, set off a violent backlash across Egypt and underscored the new government’s determination to crush the Islamists who dominated two years of free elections.

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