MANAMA, Bahrain—Bahraini security forces carried out a violent crackdown against protesters to drive them from the center of the capital Manama, and imposed a curfew across much of the city, as the island kingdom attempted to end weeks of demonstrations. 

Riot police moved in at around 7 a.m. local time, raining down volleys of tear-gas canisters onto the camps of demonstrators, who for weeks have staged a sit-in at Pearl roundabout, according to eyewitnesses. 

Witnesses...