At least 51 policemen and pilgrims were killed and more than 250 wounded when firefights erupted between rival Shia factions during a religious festival in Iraq’s holy city of Kerbala yesterday.

As gunbattles continued throughout the day the police ordered hundreds of thousands of pilgrims to leave Kerbala a day before the end of the festival and imposed a night-time curfew.

Additional security forces were rushed to the city from Baghdad and neighbouring provinces. US jets flew low overhead in a show of force requested by the Iraqi Government. The injured were taken to hospitals on handcarts because vehicles had been banned.

The fighting appeared to be the result of an increasingly vicious power struggle between the two largest Shia militias — al-Mahdi Army, which is loyal to the radical cleric Hojatoleslam Moqtada al-Sadr and the Badr Brigade, which is affiliated to the rival Supreme Iraqi Islamic Council (SIIC). There were reports last night that the fighting had spread to Sadr City, the al-Mahdi Army stronghold in northeastern Baghdad, more than 70 miles north of Kerbala.

The festival, a celebration of the birthday of the 8th-century Imam Muhammad al-Mahdi, which attracts close to a million pilgrims, has been attacked in the past by Sunni extremists using car bombs and suicide attacks. In March 2004 they killed about 100 Shias. But yesterday’s violence was between rival Shias.

The police said gunmen loyal to Hojatoleslam al-Sadr had opened fire on them in an attempt to seize control of the area around the Imam Hussein and Imam Abbas shrines, the focal point of the festival.

Aides to Hojatoleslam al-Sadr accused police linked to the Badr Brigade of beating pilgrims who were chanting his name.

The fighting broke out early in the afternoon and witnesses reported hearing gunfire and rocket-propelled grenades well into the evening. They could see columns of smoke rising from the city centre. Several hotels were said to be burning.

The Sadrists and the SIIC have been battling for supremacy in towns and cities across Iraq’s predominantly Shia south in recent months. That struggle is expected to intensify before next year’s provincial election.

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