Tinderbox

The Mahdi is simmering and waiting to explode.

On Friday, checkpoints were set up outside mosques in Samawa as the police and other security forces tried to find militiamen. The effort caused tensions that soon escalated from fistfights to brawls and finally exchanges of gunfire, according to the authorities and witnesses.

When a leading supporter of Mr. Sadr, Sayed Muhammad Khaqani, was wounded, the situation deteriorated further. At least 13 supporters of Mr. Sadr were killed, the head of the Sadr organization in Samawa said. At least five policemen were killed and three others wounded, an Iraqi official said.

On Friday night, a curfew was imposed in Samawa but was widely ignored. On Saturday, the Mahdi Army appeared to be in control of the northern half of the city.