At least four people have been killed after violence erupted overnight when Shia worshippers marched through a Sunni neighbourhood of Baghdad on their way to a shrine for an annual ritual, police said.
Mobs burned several buildings in the Sunni district of Adhamiyah which the pilgrims have to cross to reach the holy Shia site on the other side of the Tigris river.
Four people were killed in the violence, police said.
"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 news agency on Thursday.
"They burned it as well as 17 other houses," he said.
The officer said the violence broke out after 2am (11:00 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 thought they were being shot at by Adhamiyah residents.
Police sources said those who died were burnt to death trapped inside their torched homes. Medical sources confirmed four people had been killed 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 Shia Islam, who died in 799 AD.