At least eight people have been killed and 20 injured in a car bomb attack north of Baghdad, the Iraqi capital.
Officials said women and children were among those killed when the car, which was parked on the side of the road, blew up on Thursday afternoon.
Raysan Abood, an eyewitness, said the car was disguised by flags commonly flown at such commemorations and was playing mourning music, allowing it to penetrate the crowd of pilgrims.
"I saw seven dead bodies. I know them by name. They were our friends and they were delivering food and tea to the pilgrims who came from other towns," Abood said.
The blast occurred near the town of Dujail, 50km from Baghdad, on the main road to Samarra, a source in the police operations centre in Tikrit said. Eight cars were said to have been set ablaze in the explosion.
The pilgrims were heading to the golden shrine of Samarra for Friday ceremonies marking the death of the 11th imam.
Samarra, located about 100km north of the Iraqi capital, is the home of the al-Askari mosque and shrine. Two of the 12 revered Shia imams are buried there.
In February 2006, fighters attacked the al-Askari mosque, sparking waves of sectarian violence that pushed Iraq close to all-out civil war.
Dozens of people aslo died in attacks on Shia pilgrims last month near the holy city of Karbala.