Masked gunmen opened fire at Iraqi protesters in the Shiite holy city of Karbala on Tuesday, killing 18 people and wounding hundreds, security officials said, in one of the deadliest single attacks on protesters since anti-government demonstrations erupted earlier this month. The attack, which happened overnight, came as Iraqis took to the streets for a fifth consecutive day, protesting their government’s corruption, lack of services and other grievances. The bloodshed in Karbala, a major pilgrimage site where a revered Shiite figure was killed in a 7th century battle, could mark a turning point in the demonstrations.