Violence is estimated to have killed at least 300 people in Karachi this year
A suicide bomber on Thursday targeted a Karachi police officer involved in a campaign against Islamist militants, killing at least three other people, Pakistani officials said.
Senior police official Rao Anwar said he was the target of the blast, which tore through a busy area lined with market stalls at around 10:30 am (0530 GMT), and said he had been threatened by militants.
"I was moving with my squad in Malir Halt district when the explosion took place. I am unhurt, but some of my men have been injured," Anwar told AFP.
An armoured police vehicle and vendors' stalls were damaged in the explosion which police official Tahir Naveed said was a suicide attack.
Up to four kilograms (nine pounds) of explosives were used in the device, according to a bomb disposal official.
Karachi is Pakistan's largest city and its port has been a lifeline for US troops fighting the Taliban in landlocked Afghanistan, though Pakistan shut down the supply lines last November when US air strikes killed 24 soldiers.
The bomber struck near a bank in a garrison area of the sprawling city, which is the country's financial capital and home to 18 million people.
"At least four people are dead and 13 others were injured," police surgeon Hamid Parhyar told AFP.
Police suspect the bomber was among the dead. All of the dead were men and there were two teenagers among the wounded.
Anwar has been heavily involved in a campaign against Islamist militants in Karachi, which is plagued with ethnic, sectarian and political violence.
"I got several letters sent by unknown militants, the latest of them was received last month, in which they had threatened me with grave consequences if I continued to hunt their terror accomplices," he told AFP.
"Such attacks will not deter us from our mission to keep our people safe," he said.
Anwar was also involved in last December's raid on a self-styled seminary, where officers said they rescued 53 students, including children as young as seven chained up in the basement, who said they had been regularly beaten.
Madrassas, which provide the poorest families with the only education they can afford, are not tightly regulated in Pakistan and have served as recruitment grounds for the Taliban and other Al-Qaeda-linked terror groups.
The last suicide attack in Karachi killed two policemen in November.
This week, Pakistan's human rights commission said ethnic, sectarian and politically linked violence had killed 300 people so far this year in Karachi, where migration has turned the city into a melting pot of different people.
Police said three officers patrolling PIB Colony, one of the areas affected by the recent violence, were killed in a drive-by shooting on Thursday.