KARACHI, Pakistan -- A car bomb exploded outside a mosque on Sunday, killing 37 people and wounding another 141 in a Shiite Muslim dominated 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 Shiite 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 Shiite 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 100 kilograms (220 pounds) of explosives was planted in a car.
Col. Pervez Ahmad, an official with a Pakistani paramilitary force called the Rangers, said a chemical used in the blast caught fire and spread the destruction beyond the blast site. Several buildings nearby were engulfed in flames.
Men and women wailed and 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 meters (yards) wide and more than 1 meter (4 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 Shiite Muslims who make up about 20 percent of Pakistan's population of 180 million people. Sunni militants linked to al-Qaida and the Taliban view Shiites as heretics.
Tahira Begum, a relative of a blast victim, demanded the government take strict 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 Shiite 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, an ethnic group that migrated from Afghanistan more than a century ago. Most Hazaras are Shiites.
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 Shiite. Pakistan banned Lashkar-e-Jhangvi in 2001, but the group continues to attack Shiites.
According to Human Rights Watch, more than 400 Shiites were killed last year in targeted attacks across the country, the worst year on record for anti-Shiite 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 Shiites, and many Pakistanis question how these attacks can happen with such regularity.
A resident who lived in the area where the bomb went off Sunday said there had been another blast nearby 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 for several days while demanding a military-led crackdown against the Lashkar-e-Jhangvi 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 is linked to the latest violence against Shiites.
The repeated attacks have left many Shiites outraged at the government. After the last blast in Quetta, Shiites in Karachi and other cities also demonstrated in support for their brethren in Quetta. Shiites in Karachi set fire to tires and blocked off streets leading to the airport. Many Karachi residents planned to strike on Monday as a form of protest following Sunday's attack in their city.