The riot police brought a violent end to peaceful protests here on Tuesday, beating, sexually harassing and detaining some of Egypt’s most prominent human rights activists in a burst of repression that seemed likely to broaden opposition to the military-backed government. The crackdown was the highest-profile police action against non-Islamist protesters since July 3, when the military ousted President Mohamed Morsi , setting off months of bloody civil conflict between Mr. Morsi’s Islamist supporters and the state. While the deadly repression of Islamists in recent months has evoked little sympathy from the public or consternation among officials, the violence on Tuesday posed a bigger threat to the government, which has relied on support from non-Islamists. In scenes that spread on social media, activists who have played central roles in Egypt’s post-revolt struggles were manhandled and groped by officers, some in plain clothes, and shoved into police vans. […]