Iraqi authorities say several bombs exploded in Baghdad and the central city of Hilla early Wednesday, killing at least 31 people and wounding many others.

They said four bombs targeted Shi'ite pilgrims who were gathering in the Iraqi capital for an annual procession in honor of a revered Shi'ite imam. At least least 18 people were killed. Sunni insurgents in Iraq have attacked Shi'ite pilgrims repeatedly in recent years.

In Hilla, officials said two car bombs detonated near a restaurant frequented by police, killing at least 13 people. Iraqi security forces also have been frequent targets of insurgents.

Violence in Iraq has declined significantly since its peak in 2006 to 2007, but attacks remain common, especially in Baghdad.