A suicide bomber disguised as a policeman killed at least 32 people and wounded scores in an attack on Shi'ite Muslim pilgrims at a checkpoint in the southern Iraqi city of Basra on Saturday, police said.

The attack at the end of Arbain, one of the main religious observances on the Shi'ite calendar, came as a political crisis in the Shi'ite-led government renewed fears of a return to sectarian violence in the country.

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. Weeping relatives gathered at the hospital as soldiers, police and civilians rushed blood-covered victims there. Some of the wounded were stuffed into car trunks.

Now we have 32 killed and more than 100 wounded, said Ahmed al-Sulaiti, deputy head of the Basra provincial council. We are waiting for more details from the security forces about the reasons behind the attack and how they managed to do it.

A provincial health official who asked not to be named put the toll at 35 dead and 90 wounded.

Arbain has been a repeated target of militants since the U.S.-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.

Many of the incidents involved methods such as suicide bombings, the signature of Iraq's al Qaeda affiliate.

Attacks on Shi'ites have killed dozens of people since Shi'ite Prime Minister Nuri al-Maliki's government issued an arrest warrant for a Sunni vice president, triggering a political crisis that risks scuttling a power-sharing agreement.

Violence had declined since the height of sectarian slaughter in 2006-2007, when thousands were killed in intercommunal strife. But the withdrawal of the last American troops in December has fanned worries of a resurgence.

(Additional reporting by Ahmed Rasheed; Writing by Patrick Markey; Editing by Andrew Roche)