Iraqi officials say gunmen have killed at least 20 Shi'ite pilgrims as they gathered for a major religious event in Baghdad.
They say more than 300 people were injured when gunmen fired on pilgrims as hundreds of thousands of people marched to a Shi'ite shrine in Kazimiyah.
Iraqi officials say police captured at least four gunmen.
The pilgrimage commemorates the death of Imam Moussa Kadhim 12 centuries ago. He is buried in a tomb in Kazimiyah.
Iraqi authorities banned vehicles from Baghdad streets until after the pilgrimage ends Sunday evening to try to stop Sunni insurgents from targeting Shi'ites with car bombs.
During last year's pilgrimage, almost one-thousand Shi'ites died in a stampede on a bridge over the Tigris River, sparked by false rumors of a suicide bomber in the crowd.
Iraqi Industry Minister Fawzi Hariri downplayed concerns that Iraq may be entering a civil war between Sunnis and Shi'ites. Hariri, during a television interview (on CNN,) said the divisions in Iraq stem more from political ideology than from religion or ethnicity.
Democratic US Senator John Kerry said in a separate interview on the ABC network, that Iraq was already in a civil war. But Republican Senator John McCain disputed this, saying he believes it is still possible to bring things under control.