Car bombs targeting Shiite pilgrims in the Iraqi capital killed at least 32 people on Saturday, officials said, marring commemorations for a revered Shiite imam attended by tens of thousands.

An interior ministry official said a car bomb exploded at about 12:15 pm (0915 GMT) on a highway near Shuala in north Baghdad, killing 14 people and wounding 32, while a second car bomb exploded at the Aden intersection near Kadhimiyah about 2:00 pm, killing 18 people and wounding 36.

A medical source said Baghdad hospitals had received nine bodies and 47 wounded people from the first attack, and 25 bodies and 105 wounded from the second.

The attacks came as tens of thousands of Shiite pilgrims flocked to the Kadhimiyah area for the climax of commemorations marking the death in 799 of Imam Musa Kadhim, the seventh of 12 revered imams, who is said to have been poisoned.

Fadhel al-Anbari, secretary general of the Imam Kadhim shrine, said at a news conference that six million people had participated in the commemorations which began about a week ago.

The Baghdad Operations Command said in a statement that the first attack was a suicide bombing, and that security forces had arrested a man who accompanied the bomber but got out of the vehicle before the explosion.

And a police first lieutenant at the scene of the second blast said that it was also a suicide bombing that hit a minibus carrying pilgrims.

An AFP journalist said there were four burned cars and two minibuses at the scene of the first blast, one of which was completely destroyed.

Car parts were blown more than 100 metres (yards) from the site.

"We took many people out from the buses, and all of them were burned," one man who had been serving water to pilgrims said, declining to give his name.

Some people were screaming, while others appeared to be dead, he said.

"It was terrible. I will never be able to forget this scene."

"I saw the explosion and... things were thrown in every direction," said a man from one tent near the site of the second blast where food and water is served to pilgrims.

"We went to help the victims and there were many killed and wounded," he said.

The AFP journalist said there were a number of military and emergency vehicles at the scene of the second explosion, which had already been cleaned up.

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

Those attacks included a car bomb on the outskirts of Kadhimiyah that killed seven people, and another blast in Karrada in central Baghdad amid pilgrims' food tents that caused 16 fatalities.

Saturday's ceremonies began at about 8:00 am (0500 GMT) at the shrine, with a religious leader telling the story of the imam, while people in the crowd wept.

Pilgrims then marched with an empty coffin symbolising the imam, repeating religious chants, as some hit their chests and heads with their hands in a sign of mourning.

"More people are attending the commemoration this year than in 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 smeared mud on his face and his black robe in a sign of mourning.

"The imam is the symbol of martyrdom for Muslims in general, and especially for Shiites," said Abdullah Kitawi, 42, who said he walked for four days from Kut, 160 kilometres (100 miles) southeast of the city.

"Our mission is to preserve this commemoration as long as we live."

Security measures in Kadhimiyah were tight, with anti-terrorism special forces deployed. Pilgrims were searched at various points, as helicopters clattered overhead.

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

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.