The explosion occurred yesterday outside a restaurant in the Hub industrial area, which links Karachi with the troubled Balochistan province, as a vehicle carrying President's son Salman Mamnoon passed by.
Salman, who was apparently the target of the attack, escaped unhurt.
"The explosives were planted in a motorcycle parked near the restaurant and apparently the militants knew the President's son Salman would pass through on the road as he frequently goes to Hub industrial area where he has a farming business," an official told PTI.
Thirteen people, including four policemen, also injured in the attack.
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"An ASI of the Karachi police and three other policemen in Salman's convoy sustained injuries," he said.
According to television channels, the blast was so loud it was heard in nearby areas and glass windows of nearby shops were shattered as panic spread among local residents.
No group immediately claimed of responsibility for the attack.