Smoke from burning cars rises after clashes broke out Tuesday in the streets of Shiite holy city of Karbala, Iraq, 50 miles south of Baghdad. Police ordered a curfew Tuesday in Shiite holy city of Karbala and told more than 1 million pilgrims to leave after two days of violence claimed at least 27 lives during a Shiite religious festival.

Baghdad ? Riots broke out during a religious festival in the holy city of Karbala on Tuesday, prompting the evacuation of hundreds of thousands of Shiite pilgrims and leaving 28 people dead, police said.

Meanwhile, Iranian President Mahmoud Ahmadinejad predicted at a televised news conference that the authority of the U.S. military will soon collapse in Iraq. “The political power of the occupiers is being destroyed rapidly and very soon we will be witnessing a great power vacuum in the region,” Ahmadinejad said. “We, with the help of regional friends and the Iraqi nation, are ready to fill this void.”

Karbala police spokesman Raheem Imshawer said the violence in that city began after police attempted to confiscate weapons at the entrance to the area surrounding the shrines to Muhammed al-Mahdi, one of the 12 imams revered by many Shiites. Pilgrims had walked to Karbala, about 70 miles south of Iraq, from across the country to celebrate the birth of Mahdi in what was scheduled to be a two-day festival.

Iraqi television channels showed chaos erupting in the town square near the two shrines about 11 a.m., with some people firing weapons randomly while others stampeded away from the scene. Within a few hours, the pilgrims had been evacuated from the city in buses and minivans and a curfew had stopped all vehicle traffic in the area. Imshawer said that at least 145 people were injured in the clashes.

Imshawer said many of the gunmen were followers of Muqtada al-Sadr, the influential cleric whose followers make up Iraq’s largest Shiite militia, the Mahdi Army. Over the past several months, the group has battled the Badr Organization, a rival Shiite group, for control of southern Iraq, where the U.S. military maintains a minimal presence. The majority of police officers in the region are loyal to the Supreme Islamic Iraqi Council, the Shiite political group that controls the Badr Organization.

In the past few weeks, two provincial governors have been assassinated in southern Iraq, escalating tensions between the two militias. Both men were members of the Supreme Islamic Iraqi Council, the largest party in the Iraqi parliament.

Many of the estimated 1 million pilgrims ignored prohibitions against carrying weapons. Muntadar al-Jabiri, a commander in the Mahdi Army, said the fighting started because other Shiites were allowed to bring guns into the city while Sadr’s followers were not.

“Some of us were carrying pistols, but not all of us,” Jabiri said about his 31-person group. “They were authorized pistols from Sadr’s office, and we had IDs to carry them. They do not respect us just because we are from the Sadr bloc.”

In apparent retaliation for the sense of mistreatment in Karbala, Sadr supporters set fire to a Supreme Islamic Iraqi Council office in the Baghdad neighborhood of Kadhimiyah.

Also Tuesday, the U.S. military announced that troops had killed 33 Sunni insurgents who were controlling the water supply in the town of Khalis, about 50 miles north of Baghdad. The military said the Sunnis had been withholding water from the city’s Shiite residents.

According to a statement provided by the military, an airstrike killed 20 of the insurgents, while a simultaneous ground attack killed 13 others.