Dozens of people gathered at the site of the explosion and chanted anti-government slogans, the Reuters news agency reported.
Pakistan has been struck by a series of violent attacks that have killed hundreds of people in recent months, including Benazir Bhutto, a former prime minister and opposition leader who assassinated in Rawalpindi on December 27.
Last week, 16 policemen and three civilians were killed in a suicide attack in the eastern city of Lahore.
No one has claimed responsibility for the attacks but the government has blamed them on al Qaeda-linked groups based in the lawless tribal region of the country bordering Afghanistan.
Pervez Musharraf, the Pakistani president who is visiting Karachi, said earlier on Monday that militancy posed a big threat to the country and expressed his government's resolve to eradicate it.
"The president said this was a serious challenge to the country and the nation would have to fight this menace jointly and boldly," the official Associated Press of Pakistan reported.
The violence has increased security concerns and comes weeks before elections on February 18 that could weaken Musharraf's grip on power as his allies are expected to fare poorly.
The vote was meant to take place on January 8 but was postponed after Bhutto's killing.