KARACHI — A car bomb exploded outside a mosque on Sunday, killing 37 people and wounding another 141 in a Shi’ite Muslim neighborhood in the southern Pakistan city of Karachi — the third mass casualty attack on the minority sect in the country this year.
No one has taken responsibility for the bombing, but Shi’ite Muslims have been increasingly targeted by Sunni militant groups in Karachi — Pakistan’s economic hub and site of years of political, sectarian, and ethnic violence, as well as other parts of the country.
The bomb exploded outside a Shi’ite mosque as people were leaving evening prayers in Pakistan’s largest city. Initial reports suggested the bomb was rigged to a motorcycle, but a top police official, Shabbir Sheikh, said later that an estimated 220 pounds of explosives were planted in a car.
Colonel Pervez Ahmad, an official with a Pakistani paramilitary force called the Rangers, said a chemical used in the blast caught fire and spread the damage beyond the blast site. Several buildings nearby were engulfed in flames.
Ambulances rushed to the scene where residents tried to find victims buried in the rubble of collapsed buildings. The blast left a crater that was 2 yards wide and more than 3 feet deep.
‘‘I was at home when I heard a huge blast. When I came out, I saw there was dust all around in the streets. Then I saw flames,’’ said Syed Irfat Ali, a resident who described how people were crying and trying to run to safety.
A top government official, Taha Farooqi, said at least 37 people were confirmed dead and 141 more were wounded.
Sunni militant groups have stepped up attacks in the past year against Shi’ites, who form about 20 percent of Pakistan’s population of 180 million people. Sunni militants linked to Al Qaeda and the Taliban view Shi’ites as heretics.
Tahira Begum, a relative of a blast victim, demanded the government take action against the attackers.
‘‘Where is the government?’’ she asked during an interview with local Aaj News TV. ‘‘Terrorists roam free. No one dares to catch them.’’
It was the third large-scale attack against members of the minority sect so far this year. Two brazen attacks against a Shi’ite Hazara community in the 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, an ethnic group that migrated from Afghanistan more than a century ago. Most Hazaras are Shi’ites.
Pakistan’s intelligence agencies helped nurture Sunni militant groups like Lashkar-e-Jhangvi in the 1980s and 1990s to counter a perceived threat from neighboring Iran, which is mostly Shi’ite. Pakistan banned Lashkar-e-Jhangvi in 2001, but the group continues to attack Shi’ites.
According to Human Rights Watch, more than 400 Shi’ites were killed last year in targeted attacks across the country, the worst year on record for anti-Shi’ite violence in Pakistan. The 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 Shi’ites, and many Pakistanis question how these attacks can occur with such regularity.
A resident who lived in the area where the bomb went off said there had been another blast just a few months ago.
‘‘The government has totally failed to provide security to common people in this country,’’ Hyder Zaidi said.
After the Jan. 10 bombing in Quetta, the Hazara community held protests, which spread to other parts of the country. The protesters refused to bury their dead while demanding a military-led crackdown against the Lashkar-e-Jhangvi group.
The repeated attacks have left many Shi’ites outraged at the government.