Security is very tight in the Iraqi city of Karbala, as pilgrims stream into the city to mark a deeply symbolic Shi'ite Muslim holiday.

Checkpoints around the central Iraqi city are on the lookout for potential terrorists. Shi'ite militias are handling much of the security.

On Sunday night, several people were injured when a bus rammed a checkpoint. Coalition troops opened fire, and both soldiers and civilians were injured. Some reports said the bus had brake problems, but no official explanation was given yet.

By some estimates, more than a million pilgrims will arrive by Tuesday to celebrate the holiday known as Ashura. It marks the violent death of Mohammed's grandson, who founded the Shia sect more than 1,400 years ago.

Tuesday's observance, the first since the fall of Saddam Hussein's regime, carries special significance. The former regime persecuted Shi'ites and severely limited their ability to observe the holiday.