Bomb attack on Pakistan mosque kills 45
Senior city official Hashim Raza said 149 were also wounded in the blast in Karachi, the latest to target Shiite Muslims.
"It was a powerful blast on a congested Shiite-dominated area," senior police official Shabir Ahmad Sheikh.
Military offensives and US drone strikes against the Taleban in Pakistan have reduced the number of suicide attacks on government and military targets over the past year.
But Sunni groups, most prominently Lashkar-e-Jhangvi (LeJ), have escalated attacks against Shiites, who they believe are non-Muslims.
Bombings targeting Shiites have killed nearly 200 people in the city of Quetta alone since the start of the year.
In 2012, sectarian attacks and clashes climbed by 47 percent to 208, according to the Pak Institute for Peace Studies, a prominent Pakistani think tank.
While the Quetta carnage grabbed world attention, a Reuters inquiry into a lesser known spate of murders in Karachi suggests the violence is taking on a volatile new dimension as a small number of Shiites fight back.
Pakistan's Western allies have traditionally been fixated on the challenge posed to the nuclear-armed state by Taleban militants battling the army in the highlands on the Afghan frontier.
But a cycle of tit-for-tat killings on the streets of Karachi points to a new type of threat: a campaign by LeJ and allied Pakistani anti-Shiite groups to rip open sectarian fault-lines in the city of 18 million people.
Pakistani intelligence agents say the LeJ has become a major security threat in Pakistan, which is also struggling with a fragile economy, dilapidated infrastructure and widespread poverty.
Shiite frustrations are rising with each blast. Shiites fired weapons in the air on Sunday night in Karachi, a bustling metropolis plagued by ethnic and political violence and crime.
"The explosion was so massive it jolted the entire area," said witness Ali Reza. "Two flats and nearby shops caught fire after the explosion and balconies of various buildings collapsed."
Another witness, Muhammad Kazim, said women and children who were shopping nearby were wounded.