BAGHDAD (Reuters) – A roadside bomb exploded in northern Baghdad on Sunday, killing two Iraqis and wounding 11 others taking part in a long pilgrimage on foot to mark a major Shi’ite Muslim religious rite, police said.

The pilgrims were in a group heading for the southern city of Kerbala where millions gather each year for Arbain, one of the holiest events for Iraq’s majority Shi’ites.

Arbain is the end of a 40-day mourning period after Ashura, a religious ritual that commemorates the death of the Prophet Mohammad’s grandson Imam Hussein in 680.

Early last month, a suicide bomber killed at least 35 pilgrims near the Imam Kadhim shine in northern Baghdad.

Almost six years after the U.S.-led invasion unleashed years of bloodshed, violence dropped sharply in 2008 and local security forces are taking greater control as U.S. forces prepare to withdraw by the end of 2011.

The U.S. military says Iraq’s ability to ensure the safety of pilgrims in Arbain will be another major challenge after the January 31 provincial elections that were held without a single major militant attack.

Arbain brings huge crowds of Shi’ite, mainly in black and carrying colorful flags and banners. Pilgrims are particularly vulnerable as they make the long walk to Kerbala, 80 km (50 miles) south of Baghdad.