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BASRA, Iraq, Jan 18 (Reuters) - Iraqi police clashed with gunmen from a Shi'ite cult who staged a series of hit-and-run raids in two southern cities on Friday, security officials said.

Witnesses in Iraq's southern oil hub of Basra and Nassiriya said at least four people may have been killed in the violence, in which gunmen were reported to be using heavy machineguns and mortars.

The clashes came as religious observations for the annual Ashura festival, one of the holiest events in the Shi'ite Muslim religious calendar, approached their peak across southern Iraq.

Police in Basra and Nassiriya said fighters from the "Soldiers of Heaven" cult, an obscure group once led by a man who claimed to be the mahdi, an Islamic messiah-like figure, had opened fire on security forces in both cities.

In central Basra, a Reuters cameraman said he saw the bullet-riddled body of one man lying in a street, as well as a policeman who had been shot in the arm.

A policeman in Nassiriya, 375 km (235 miles) southeast of Baghdad, said three policemen may have been killed in clashes there but the reports could not be verified immediately.

Police said a curfew had been declared in the city.

Basra police chief Major-General Abdul Jalil Khalaf said police and Iraqi soldiers had responded to several attacks across the city.

"We are controlling the situation. I don't have any information about casualties at the moment," Khalaf said.

"They have been attacking security forces and disappearing," he told Reuters. The Reuters cameraman in Basra said he also saw about 30 gunmen dressed in black carrying semi-automatic weapons and rocket-propelled grenade launchers. Some of them were driving at least two vehicles seized from police, he said.

Police said the gunmen were supporters of Ahmed Hassani al-Yemeni, who took over after the cult's previous leader was killed in a battle with his followers a year ago.

A man who said he was from the movement told Reuters in Basra that their fighters had decided to attack security forces because of persecution he said the cult had suffered. He also said they believed the mahdi would appear on Friday.

The previous leader, who used the name Mahdi bin Ali bin Ali bin Abi Taleb, had claimed to be the mahdi.

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