Shia pilgrims gather at the holy shrine of the Imam Moussa al-Kadhim during the annual commemoration of the saint's death in Baghdad. The annual gathering has been a target for bombers. (Karim Kadim/Associated Press)

Car bombs targeting religious processions in Iraq's capital killed at least 26 people Saturday on the last day of a Shia pilgrimage already hit three times in some of the deadliest violence since American troops withdrew, police said.

The blasts were latest in a fierce wave of terror attacks on the annual pilgrimage that sees hundreds of thousands visit the shrine to an eighth-century Shia imam in northern Baghdad. On Wednesday, 72 people died in nearly two dozen co-ordinated bombings targeting pilgrims across the country. Al-Qaeda's affiliate in Iraq on Saturday claimed responsibility for that attack.

Saturday's explosions raised the week's death toll to more than 100 and cast further doubt on the divided government's ability to secure the country after the American withdrawal.

Al-Qaeda has been unleashing attacks every few weeks, trying to weaken the government of Prime Minister Nouri al-Maliki and spark another round of the sectarian violence that brought Iraq to the brink of civil war only a few years ago.

While fighting between Sunni and Shia Muslim communities has all but died out in recent years, al-Maliki's government has been plagued by sectarian tension since before the last American troops withdrew six months ago.

The first car bomb exploded just after noon near a throng of pilgrims streaming through the Shiite neighbourhood of Shula in the north of the city, heading toward the shrine to Imam Moussa al-Kadhim. At least 14 people including two policemen were killed in the blast and 46 others were wounded, police said.

The bomb was hidden in a taxi parked among a group of other taxis waiting along the procession route to take pilgrims back to their home cities once Saturday's ceremonies were over, the police official said. He spoke on condition of anonymity because he was not authorized to release the information. A hospital worker confirmed the death toll, also speaking on condition of anonymity for the same reason.

An hour later, a second car exploded in Kadimiyah neighbourhood, about five kilometres from the mosque that contains the shrine where al-Kadhim is said to be buried. The same police official said that 12 people were killed and 26 were wounded when a parked car exploded as the crowds passed by.

'Dysfunctional political process'

The bombers' ability to penetrate so close to the shrine indicates the challenges faced by Iraq's security forces in securing huge religious gatherings.

"Those behind the attacks, they've become more determined now and see more of an opportunity because of the dysfunctional political process," said Salman Shaikh, director of the Brookings Doha Center and an analyst on regional politics.

The Kadimiyah attack hit inside a tight ring of security that authorities had thrown up around the shrine's surrounding neighbourhood after Wednesday's multiple bombings. Police and soldiers had been searching vehicles and people at checkpoints leading into Kadimiyah for days.

"There is no real security, no real searches," said Mohamed Ali, who witnessed the first bombing in Shula, where authorities were towing away the burned remains of a car. 

Despite the latest violence, Saturday's commemorations continued for al-Kadhim, a revered imam who was the Prophet Muhammad's great-grandson.

Massive crowds carried symbolic coffins through the streets and pilgrims beat their chests as a sign of mourning as they streamed through Baghdad toward the mosque's two golden domes.

The al-Kadhim procession was struck by tragedy in 2005, when thousands of Shiite pilgrims panicked by rumours of a suicide bomber broke into a stampede on a bridge, leaving some 1,000 of them dead. Police later said no explosives were found on the bridge, and poor crowd control appeared largely to blame.