Police ordered hundreds of thousands of pilgrims to leave the Iraqi city of Kerbala on Tuesday as a battle raged between Iraqi security forces and gunmen near two of Shi'ite Islam's holiest shrines.

A senior security source in Baghdad said 25 people had been killed, mostly policemen. The director of Kerbala's al-Hussein Hospital said it had received eight bodies and 29 wounded.

Interior Ministry spokesman Brigadier-General Abdul Kareem Khalaf told state television that reinforcements were being rushed to Kerbala from Baghdad and neighbouring provinces. He said 50 people had been killed and wounded in the violence.

The fighting appeared to be between gunmen loyal to the fiery Shi'ite cleric Moqtada al-Sadr, possibly members of his Mehdi Army militia, and police linked to the rival Shi'ite political movement, the Supreme Iraqi Islamic Council (SIIC) and its Badr Organisation.

Police said a curfew had been imposed and pilgrims ordered to leave the city, 110 km (68 miles) south of Baghdad, bringing to an abrupt halt a major Shi'ite festival that was to have run for two days until Wednesday.

"They came in big buses and now police are forcing them to return on these buses," said one local police official.

Many of the pilgrims had also walked from Baghdad and elsewhere to mark the 9th century birth of Mohammad al-Mahdi, the last of 12 imams Shi'ites revere as saints.

Iraq's security forces had originally feared that Sunni Islamist al Qaeda might try to launch a large-scale attack on the pilgrims to inflame sectarian tensions.

Reuters witnesses in the city could hear the sound of intense gunfire and rocket-propelled grenades being fired and saw columns of smoke rising from the centre of the city, apparently from cars that police said had been set ablaze.

Police said gunmen armed with automatic weapons and pistols had tried to take control of the area around the Imam Ali and Imam Abbas shrines, the focal point of Tuesday's ceremonies.

Hazem al-Araji, a senior aide to Sadr, told Reuters from Kerbala that the clashes erupted when police objected to pilgrims chanting pro-Sadr slogans and began beating them.

Another Sadr aide, Ammar al-Saidi, said the treatment of the pilgrims had enraged Sadrists in Kerbala and sparked revenge attacks on the security forces.

In a sign of how the violence could spread, police said Sadr supporters set fire to a SIIC office in the Shi'ite district of Kadhimiya in Baghdad.

In Kerbala, a Reuters witness saw gunmen roaming the streets armed with heavy machineguns, rocket-propelled grenade launchers, batons and swords, beating pilgrims, including women.

The Sadrists and SIIC, the two biggest Shi'ite blocs in parliament, are locked in a power struggle for control of towns and cities in Iraq's predominantly Shi'ite south. The police in many of these towns are seen to be loyal to Badr.

Analysts fear the struggle for dominance will intensify ahead of provincial elections expected to take place next year.

The fighting is likely to be seen as embarrassing for Prime Minister Nuri al-Maliki, who wants to show that his security forces can take control of security from US-led forces.

One of his two deputies, Barham Salih, warned in an interview with Reuters late on Monday that an early U.S. troop pullout would trigger a full-scale civil war.

"A premature withdrawal of troops from Iraq will be a disaster," Salih said.

"It will lead to an all-out civil war, it will lead to a regional war in my opinion because the fate of Iraq is crucial to the regional balance and to regional security."    

US jets were dispatched to fly over Kerbala as a "show of force" at the request of Iraqi authorities, but that was the only US participation so far in the operation, said Major Alayne Conway, spokeswoman for US forces south of Baghdad.

Tuesday's vio