Iraqi authorities have tightened security in Baghdad in an effort to prevent attacks on Shi'ite pilgrims as they make their way to the capital to commemorate the death of a revered imam.

The pilgrims are often targets as they journey to a shrine in honor of Imam Moussa Kadhim, who died 12 centuries ago.

Despite the additional security measures, police say at least 18 people were wounded Thursday when a bomb exploded as pilgrims headed toward the shrine in northern Baghdad's Kazimiyah district.

There are conflicting reports about whether the explosive was a roadside bomb that struck a bus carrying pilgrims or a bomb planted on the pilgrims' vehicle.

Tens of thousands of pilgrims are heading to Baghdad, and the number is expected to reach its peak Saturday.

Violence has marred this religious observance in years past.

In 2006, snipers killed at least 20 Shi'ites in Baghdad as they walked to the shrine where Kadhim is buried.  And, at the same festival in 2005, about 1,000 pilgrims died in a stampede over a bridge triggered by rumors of a suicide bomber.

Some information for this report was provided by AP and Reuters.