Deadly violence erupted overnight when Shiite worshippers marched through a Sunni neighbourhood of Baghdad on their way to a shrine for an annual ritual, police said Thursday.

Mobs burned several buildings in the Sunni district of Adhamiyah which the pilgrims have to cross to reach the holy Shiite site on the other side of the Tigris river.

"Some elements infiltrated in the midst of the pilgrims heading to the shrine of Imam Musa Kadhim attacked a Sunni Waqf (endowment) building," a police colonel told AFP.

"They burned it as well as 17 other houses," he said.

The officer said the violence broke out after 2:00 am (1100 GMT) when members of the crowd spread a rumour that a suicide bomber was among them.

He said police fired shots in the air in an attempt to contain the panic but the violence only swelled because the crowd through they were being shot at by Adhamiyah residents.

Police sources said four people were burnt to death trapped inside their torched homes. Medical sources gave the same toll but said some of the victims had bullet wounds.

The Iraqi state has deployed 75,000 members of the security forces to protect the droves of worshippers who have been walking for days towards the shrine in northwestern Baghdad.

It is the site of a shrine dedicated to Imam Musa Kadhim, the seventh of 12 revered imams in Shiite Islam, who died in 799 AD.

At least seven people were killed in a car bomb attack against worshippers on Saturday and another six were killed in two attacks on Tuesday.

Calm had returned to the area by dawn on Thursday and, according to state TV, the interior ministry arrested several people suspected of involvement in the overnight violence.

Prime Minister Haider al-Abadi visited both neighbourhoods early Thursday to review the security set-up.

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