Police say a car bomb has killed at least 37 people and wounded another 141 in a neighborhood dominated by Shia Muslims in the southern Pakistan city of Karachi.

The bomb exploded outside a Shia mosque as people were leaving evening prayers, said police official Azhar Iqbal. Men, women and children were among those killed and wounded, he said.

Fifty others were wounded, said a top government official, Taha Farooqi. He said some people were feared trapped in the rubble of buildings that collapsed in the bombing.

Initial reports suggest the bomb was rigged to a motorcycle, although a survey of the damage indicates there could have been additional explosives planted at the scene, the police official said.  Farooqi said several buildings nearby had caught fire.

"I heard a huge blast. I saw flames," Syed Irfat Ali, a resident of the area, said, adding that people were crying and running to safety.

No one has claimed responsibility, but Sunni militants linked to al-Qaeda and the Taliban have targeted Shia in the past, claiming they are heretics.

Sunni militant groups have stepped up attacks in the past year against Shia Muslims who make up about 20 per cent of Pakistan's population of 180 million people.

Bombs aimed at busy areas

Two brazen attacks against a Shia Hazara community in southwestern city of Quetta killed nearly 200 people since Jan 10. Pakistani militant group Lashkar-e-Jhangvi claimed responsibility for the bombings, which ripped through a billiard club and a market in areas populated by Hazaras, which are mostly Muslim Shia.

Pakistan's intelligence agencies helped nurture Sunni militant groups like Lashkar-e-Jhangvi in the 1980s and 1990s to counter a perceived threat from neighbouring Iran, which is mostly Shia. Pakistan banned Lashkar-e-Jhangvi in 2001, but the group continues to attack Shia.

According to Human Rights Watch, more than 400 Shia were killed last year in targeted attacks across the country, the worst year on record for anti-Shia violence in Pakistan. The human rights group said more than 125 were killed in Baluchistan province. Most of them belonged to the Hazara community.

Human rights groups have accused the government of not doing enough to protect Shia.

After the Jan 10 bombing, the Hazara community held protests, which spread to other parts of the country. The protesters refused to bury their dead for several days while demanding a military-led crackdown against the Lashkar-e-Jhanvi group. Pakistan's president dismissed the provincial government and assigned a governor to run Baluchistan province.

No operation was launched against the militant group until another bombing in February killed 89 people.

The government then ordered a police operation and has said some members of the group have been arrested. One of the founders of the group, Malik Ishaq, was among those detained and officials said he could be questioned to determine if his group's is linked to the latest violence against Shia.