A huge car bomb blast in a mainly Shiite Muslim area of Karachi on Sunday killed at least 45 people, officials said, amid a spate of sectarian violence that has come as Pakistan prepares for elections.
The explosion blew the front off two five-storey apartment blocks in the Abbas town area of Pakistan's largest city, setting one on fire and trapping people beneath piles of rubble.
Karachi is plagued by sectarian, ethnic and political violence, with more than 2,200 people killed in shootings and bombings last year, but bomb blasts on this scale are rare.
"At least 45 people have been killed and 150 injured. The death toll may rise because the condition of half of the injured is severely critical," Hashim Raza Zaidi, the top government official in Karachi told AFP.
Hundreds of shops and houses were badly damaged in the blast, which came as worshippers were leaving nearby mosques, and the balconies of the apartment blocks were destroyed.
In Patel hospital, where many wounded were taken for treatment, a weeping mother frantically searched for her son.
"He was standing in the balcony, where is he?" she cried.
Ijaz Ali was on the third floor of one of the blocks of flats with his wife and two sons when the bomb went off.
"All of a sudden I heard a huge blast and we thought the building was going to collapse -- it was like an earthquake," he told AFP from hospital.
"The windows of my flat exploded towards me, something hit my head and knocked me unconscious. I opened my eyes in hospital and I am just relieved that my family survived."
Suresh Kumar, the health secretary for Sindh province told AFP that more than 40 were killed in the blast and the death toll could rise.
Abbas town is dominated by Shiites, who make up around 20 percent of Pakistan's 180 million population, but officials said the victims of the blast also included members of the majority Sunni group.
Violence against Shiites, who make up around 20 percent of Pakistan's 180 million population, has reached record levels, raising serious questions about security as the nuclear-armed country prepares for elections due by mid-May.
More than 400 Shiites were killed in sectarian attacks in 2012, according to rights groups, and two deadly bombings targeting the minority in the southwestern city of Quetta have already killed nearly 200 this year.
On Monday a bomb at a Sufi shrine regularly visited by Shiites in southern Shikarpur district, some 400 kilometres (250 miles) northeast of Karachi, killed two people and wounded 10 others.
A day later the Supreme Court ordered the authorities to come up with a strategy to protect Shiites after a wave of bloody attacks in the southwest.
Two major bombings in the space of five weeks targeting Shiite Hazaras in Quetta, capital of Baluchistan province -- which has been a focus of sectarian violence -- killed nearly 200 people.
Both attacks, the most recent on February 16, were claimed by the banned extremist Sunni group Lashkar-e-Jhangvi, and have highlighted the government's inability to stem sectarian violence.
The Pakistani Taliban have also increased their campaign of violence in recent months, leading to fears that it could mar a general election scheduled to take place by mid-May.
Last month the group proposed talks with Islamabad but the government insists the militants must declare a ceasefire before coming to the negotiating table -- a condition militants have rejected.