Hundreds of thousands of pilgrims have been ordered to leave a Shia festival in the Iraqi city of Karbala after heavy shooting broke out in the city.
Shias gather each year to mark the birthday of the Imam Mahdi
Gunmen exchanged fire with police near the holy Shia shrine of Imam Hussein, causing panic among worshippers.
Iraqi police say 52 people have been killed and 205 injured, mostly pilgrims or Iraqi security forces, since clashes erupted on Monday evening.
Police have stopped people entering Karbala and have sealed off the shrine.
An indefinite curfew has also been imposed on the old city, which houses the holiest shrines, and the predominantly Shia cities of Najaf and Hilla also have curfews in place.
A member of Karbala city council told the Associated Press news agency that the latest fighting had sparked pandemonium, sending pilgrims running in all directions to escape.
One pilgrim in Karbala contacted the BBC website to describe what he could see and hear from his hotel opposite the shrine of Imam Hussain.
"Shots are being fired everywhere including at hotels. We have recently seen hotels going up in flames due to rockets being fired at them by the militants," he said.
"We cannot tell who is behind this. If we try to look down to see what is going on from our hotel rooms they tell us to close the curtains. We are not allowed to leave the hotel and the shrines have been closed down."
Reinforcements being sent
Interior ministry spokesman Major General Abdul Karim Khalaf told the BBC that "the next two to three hours will be decisive" in the confrontation between the security forces and gunmen, though he maintained that the security forces were now in full control.
Maj Gen Khalaf said entry and exit points into the area had been secured and that military reinforcements were being sent to the holy city.
There have been reports that the gunmen include members of the Mehdi Army militia, loyal to Shia cleric Moqtada al-Sadr, but Maj Gen Khalaf declined to speculate on who the gunmen were, describing them repeatedly as lawless criminals who had been paid to undertake the attack.
Shias flock to Karbala each year to mark the birthday of the 9th Century Imam Mahdi, the last of 12 imams whom Shias venerate as saints, believing that he never died and will return to Earth to save mankind.
The celebrations had been due to reach their climax on Tuesday evening.
Tight security has been put in place because Sunni insurgents often launch attacks on such festivals.
Violence began on Monday night when Shia pilgrims became angry at delays caused by the strict security measures.
According to witnesses, as scuffles broke out among the restive crowd, police opened fire killing at least five and wounding dozens.
But the BBC's Mike Wooldridge, in Baghdad, says that the motive for the latest clashes is not yet clear.
At least one pilgrim was also killed on Monday by gunmen taking shots at people driving to the festival.
Another nine people were killed on Monday, when a suicide bomber blew himself up after evening prayers in a mosque in the town of Falluja, west of Baghdad.
The violence 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.