A suicide bomber disguised as a policeman killed at least 53 people and wounded scores in an attack on Shi'ite Muslim pilgrims at a checkpoint on Saturday.

The bombing at the end of Arbain, one of the main religious observances in the Shi'ite calendar, was the worst such incident this year, amid a political crisis and renewed fears of a resurgence of sectarian violence.

"A terrorist wearing a police uniform and carrying fake police I.D. managed to reach a police checkpoint and blew himself up among police and pilgrims," said a police official at the scene of the bombing.

The pilgrims had been on their way to a major Shi'ite mosque to the west of Basra, police said.

Security forces sealed off the main hospital in Basra, fearing further attacks as soldiers, police and civilians rushed blood-covered victims to the hospital. Some of the wounded were stuffed into car trunks.

Hundreds of wailing relatives packed into the city morgue searching for casualties. One woman lay on the floor screaming for her dead son and covering her head with dust from the ground in a traditional expression of grief.

Riyadh Abdul-Ameer, director of the Basra health office, told Reuters the blast killed 53 people, with another 130 wounded. Other officials had earlier put the toll at 32 to 35 dead with 90 to more than 100 wounded.

Tensions in Iraq have wider implications in the region. Neighboring Syria is in turmoil, and Shi'ite Iran and Sunni Arab Gulf nations and Turkey are positioning themselves for more influence.

Arbain has been a frequent target of militants since the US-led invasion that ousted Sunni dictator Saddam Hussein.

Scores of people have been killed in attacks on pilgrims in the last few weeks, including a suicide bombing which killed at least 44 people.-Reuters