Iraqi officials say two car bombings targeting Shi'ite pilgrims killed at least 40 people and wounded 145 on the outskirts of the holy city of Karbala.
The attacks took place Friday as hundreds of thousands of Shi'ite pilgrims converged on Karbala, south of Baghdad, for an important religious observance, the Arbain holy day, marking the end of the mourning period for the death of the 7th-century Shi'ite cleric Imam Hussein.
Details of the incident are uncertain. Some officials claim at least one of the blasts was caused by mortar fire.
Friday's attack was the third this week against Shi'ite pilgrims, frequently targeted by Sunni insurgents.
U.S. Secretary of State Hillary Clinton condemned the violence, saying in a statement that it exposes the "cynical immorality of the terrorists."
The unrest also underscores political tension over efforts to ban a group of politicians from participating in upcoming parliamentary elections.
Some Sunni politicians accuse Iraq's Shi'ite-led government of blacklisting candidates to consolidate power.