A car bomb exploded outside a mosque in Karachi on Sunday, killing dozens of people and wounding more than 140 in a Shia Muslim dominated neighbourhood.
No one has taken responsibility for the bombing, but Shia Muslims have been increasingly targeted by Sunni militant groups in the city, 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 Shia 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.
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 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 metres wide and more than 1 metre 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 wounded. The Reuters news agency put the toll given by a city official at 45 dead and 149 wounded.
Sunni militant groups have stepped up attacks in the past year against Shia Muslims who make up about 20% of Pakistan's population of 180 million people. Sunni militants linked to al-Qaida and the Taliban view Shias 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 attacks against a Shia Hazara community in Quetta killed nearly 200 people.
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 Shias.
According to Human Rights Watch, more than 400 Shias 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 Shias, and many Pakistanis question how these attacks can happen with such regularity.
A resident who lived in the area where the bomb went off on 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 bombing in Quetta on 10 January, 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 Shias.
The repeated attacks have left many Shias outraged at the government. After the last blast in Quetta, Shias in Karachi and other cities also demonstrated in support for their brethren in Quetta. Shias 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.
