Bilal Shaikh, a close aide of the president and head of presidential security who had survived three previous attempts on his life, died in the blast Wednesday after he stepped out of his armored vehicle to buy fruits from a vendor in the port city, which has been wracked for years by sectarian, militant and political violence.
The state-run Associated Press of Pakistan, quoting police, said: "As he stepped out of the vehicle, the blast occurred killing him and one of his companions as well as the fruit vendor."
The blast also injured eight people, including policemen, the report said.
Police superintendent Raja Umer Khattab told CNN preliminary investigation indicated the attacker was on foot and that those injured included policemen who were traveling in vehicles accompanying Shaikh's vehicle.
"It's not known if Mr. Shaikh had received any threats, but he was definitely the target of this attack," the police official told CNN.
Geo News quoted police as saying it was a suicide attack and that the attacker detonated his explosives vest when the Shaikh's vehicle stopped at the fruit stall.
Both Zardari and Pakistani Prime Minister Nawaz Sharif condemned the attack. Zardari's Pakistan People's Party announced a day of mourning Thursday across Sindh province of which Karachi is the capital.
Pakistan's Express Tribune said there had been three previous attempts on Shaikh's life.
No group immediately claimed responsibility for the attack and investigators so far have no suspects connected with the attack.
"Usually, the Taliban use suicide bombers to attack their targets. However, we cannot say anything about the possible perpetrators of this attack or its motives since our investigation is ongoing," a senior law enforcement official told the Tribune.
Prime Minister Nawaz Sharif, speaking at a meeting of lawmakers from Pakistan's tribal regions, vowed to eradicate terrorism, Geo News reported. Sharif became prime minister for the third time after his party won the elections in May.
"Pakistan is passing through critical circumstances, therefore, we all have to join hands to fight against the menace of terrorism," Sharif was quoted as saying.