Fierce gun battles have cut short a major religious festival in Iraq, which had drawn hundreds of thousands of Shia pilgrims to the holy city of Karbala.
Shias gather each year to mark the birthday of the Imam Mahdi
The clashes have left more than 50 dead and 200 injured, police said. Troop reinforcements have been brought in and worshippers ordered to leave the city.
Militiamen loyal to the radical Shia cleric, Moqtada Sadr, have been widely blamed for the disturbances.
But Moqtada Sadr denied any involvement and appealed for calm late on Tuesday.
The radical cleric also called for an independent inquiry into the clashes and urged his supporters to co-operate with the authorities "to calm the situation down".
In the capital, Baghdad, Iraqi police said five people were killed in clashes in the Shia stronghold of Sadr City.
Curfews have also been ordered in the mainly Shia cities of Najaf and Hilla.
Chaos and bloodshed
Troops were rushed to Karbala after the eruption of fierce fighting close to two of the most important shrines in Shia Islam, and celebrations that had brought pilgrims to the city from across Iraq and further afield turned to chaos, confusion and bloodshed.
Gunmen with rocket-propelled grenades and automatic weapons forced their way past checkpoints and appeared to be trying to take control of the area around the shrines.
As security forces fought back, several hotels were set ablaze.
Firing continued after dark when the celebrations to mark the birth anniversary of the 12th and last Shia imam should have been approaching their climax.
But the pilgrims had been ordered to leave several hours earlier.
Various sources blamed the attack on the Mehdi army militia loyal to Moqtada Sadr. In Karbala, the police are linked to their political rivals, the Supreme Iraqi Islamic Council (SIIC).
There has been apparently related trouble between Mehdi army members and supporters of the SIIC at several places in Baghdad. In one clash the police say five people were killed.
Violence began on Monday night when Shia pilgrims became angry at delays caused by the strict security measures, in place to try to prevent attacks by Sunni insurgents.
It comes just two days after Iraqi Prime Minister Nouri Maliki announced a deal between Iraqi Shia, Sunni and Kurdish politicians aimed at building national unity.
US President George W Bush welcomed the deal, but warned that much work remained to quell sectarian violence in the country.