Up to two million pilgrims are expected to gather on Saturday in the city of Karbala for the festival's climax.
Tensions are high after dozens died in clashes on Friday in two southern cities between police and a group thought to be linked to a Shia cult.
Clashes between Iraqi troops and the cult last Ashura left 263 people dead.
Friday's clashes in Nasiriya and Basra broke out after apparently co-ordinated assaults were launched on Shia worshippers and security forces by attackers thought to be linked to the Soldiers of Heaven cult.
The violence sparked fears of further attacks during the Ashura festival, which marks the killing of Imam Hussein, grandson of the Prophet Muhammad, in 680 AD.
Saturday, 19.January.2008