Tens of thousands of pilgrims packed northern Baghdad on Saturday for the peak of commemorations of the death of a revered Shiite imam, amid tight security measures after a wave of attacks killed 72.

The ceremonies began about 8:00 am (0500 GMT) at the shrine of Musa Kadhim, who died in 799 and is said to have been poisoned, with a religious leader telling the story of the imam, while people in the crowd wept, an AFP journalist said.

Pilgrims then carried an empty coffin symbolising the imam to the shrine in the Kadhimiyah area of north Baghdad, repeating religious chants, with some hitting their heads and chests with their hands in a sign of mourning.

"More people are attending the commemoration this year than previous years," said Karim Mohammed, 52. "When we ... see that there are explosions, it becomes a challenge to us, so those who decided not to go will go."

Mohammed, who said he had walked for three days from Samarra, 110 kilometres (70 miles) north of Baghdad, had put mud on his face and on his black robe in a sign of mourning.

Security measures in Kadhimiyah were tight, with anti-terrorism special forces deployed in the area. Pilgrims were searched at various points, while helicopters flew overhead.

On Wednesday, 72 people were killed and more than 250 wounded in bomb and gun attacks across Iraq, with Al-Qaeda's local affiliate, the Islamic State of Iraq, claiming responsibility.

A car bomb exploded on the outskirts of Kadhimiyah on Wednesday, killing seven people, while another bomb blasted pilgrims' food tents in Karrada in central Baghdad, leaving 16 people dead.

Shiite pilgrimages were prohibited under the rule of Saddam Hussein, but they have attracted huge numbers of people in the years since his 2003 overthrow.

Along with the security forces, the Shiite majority in Iraq has been a main target of Sunni Arab armed groups since the fall of Saddam's Sunni-dominated regime.

Violence has declined dramatically since the 2006-2007 peak of sectarian bloodshed, but attacks remain common, especially in Baghdad. A total of 132 Iraqis were killed in May, official figures show.