Iraqi forces were on alert Sunday in the city of Karbala as hundreds of thousands of Shia worshippers gathered for the annual Ashura religious festival.

It commemorates the killing of the Prophet Mohammed's grandson, Imam Hussein, in the 7th century.

A young boy flagellates himself at the Shia Karbala Mosque for the Ashura festival on Sunday.
(Khalid Mohammed/Associated Press)

Under Saddam Hussein, a Sunni, the festival was outlawed. Now, it draws huge numbers of Shias from Iraq and other countries, but also requires heightened security.

At least 8,000 police will be on duty in Karbala, where security authorities have warned hotel owners not to accept any non-Iraqi Arabs or Iraqis without identity documents and to inform authorities about suspicious individuals, said police spokesman Abdul Rahman Mishawi.

Ashura has in the past been marred with outbreaks of Shia-Sunni violence across the Muslim world. In 2004 and 2005, suicide bombings by al-Qaeda in Iraq killed at least 230 people during the ceremonies in Karbala.

As the festival began to unfold in the city 80 kilometres south of Baghdad, there was more violence in Iraq.

Mortar shells rained down Sunday on a girls' secondary school in a mostly Sunni area of western Baghdad, killing five students and wounding 20, witnesses and police said.

At least seven other people died in a series of bombings and shootings across the capital, mostly in Shia areas.

Elsewhere, Iraqi troops backed by U.S. helicopters battled insurgents Sunday 19 kilometres northeast of the Shia holy city of Najaf, Iraqi officials said.

Provincial Gov. Assad Sultan Abu Klil said a U.S. helicopter went down during the fighting, but U.S. officials would not confirm the report.

The 10-day Ashura mourning period culminates on Tuesday with processions and ceremonies at the city's sacred shrine, the burial ground of Hussein. His death in 680 AD, during a battle in Karbala for leadership of the Islamic faith, was a key historical event in the ongoing sectarian rift between Sunnis and Shias in Iraq.

Shias regard Hussein and his family as direct descendants of the Prophet, a view that has for centuries put them at odds with Sunni Muslims.

Before the fall of Saddam Hussein's regime in 2003, the minority Sunnis were in control in Iraq, but the majority Shias gained control after the U.S.-led invasion.

Border crossings closed

Iran shut down several border crossings into Iraq in advance of Ashura.

Tehran said it wants to prevent a large number of pilgrims from flocking across the border without legal documents.

An Iranian border official said checkpoints will remain open for food dealers.

Bombings, shelling and gunfire have claimed the lives of dozens in the past two days in the Iraqi capital alone.

Four people were killed Sunday by a car bomb in an outdoor market in the predominantly Shia Sadr City. On Saturday, a pair of suicide bombings killed at least 13 people in another Shia neighbourhood market in Baghdad.

On Friday, a pet market in the heart of Baghdad was bombed, killing 15 people.