The attack happened Friday at the office of the Awami National Party in western Karachi, the BBC reported.
Bashir Jan, the ANP National Assembly candidate, was the likely target of the attack, officials said. He survived the explosion.
The Taliban claimed responsibility for the attack, which killed 10 people and injured as many as 50 others.
This was the third attack by militants in Karachi in four days, Dawn News reported.
On Thursday, a bomb blast near a Muttahida Quami Movement Party office in North Nazimabad killed five people and injured 10 others, officials said.
Militants detonated an improvised explosive device near a meeting of political figures on a Karachi street corner Tuesday as well. At least five people were killed in that attack and 15 others were injured, Dawn said.
The Taliban claimed responsibility for those attacks as well.