A suicide bomber disguised as a policeman killed at least 20 people in an attack that targeted Shi'ite Muslim pilgrims passing through a checkpoint in Iraq's southern city of Basra Saturday, police said.

The attack at the end of Arbain, one of the main religious observances on the Shi'ite calendar, occurred at a time when a political crisis in Iraq's Shi'ite-led government has 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.

At least 20 were killed, including 11 policemen, and at least 70 people were wounded when the bomber detonated his explosives among pilgrims trying to pass through the checkpoint on their way to a major Shi'ite mosque to the west of Basra, police and hospital sources said.

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

Security forces sealed off entrances to the main hospital in Basra, fearing further attacks.

Weeping relatives gathered at the hospital as soldiers, police officers and civilians rushed blood-covered victims to the facility in pick-up trucks and cars. Some of the wounded were stuffed into car trunks.

Arbain has been a frequent 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 attacks on pilgrims have used methods such as suicide bombings that are the signature of Iraq's al Qaeda affiliate.

Attacks targeting Shi'ites have also 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.

Attacks had ebbed since the height of sectarian slaughter in 2006-2007 when thousands were killed in intercommunal violence among Shi'ite and Sunni. But the withdrawal of the last American troops in December has fanned worries of a spike in violence.

(Additional reporting by Ahmed Rasheed, Writing by Patrick Markey, Editing by Matthew Jones)