"We have received 11 dead bodies and several injured people," said doctor Semi Jamali, head of the emergency department at Jinnah Hospital in Karachi.
The target was an election candidate of ANP (Awami National Party). He was travelling in his car when the bomb exploded," senior police official told.
The target of the attack, Amanullah Mehsud, escaped unhurt, police official said.
Taliban spokesman Ehsanullah Ehsan claimed responsibility.
"We proudly claim responsibility for this attack, we carried it out and will carry out more of the same," Ehsan said of the first deadly attack on polling day.
The election marks the first democratic transition of power in the nuclear-armed state but the run-up to the polls has been marred by Taliban attacks that have killed more than 120 people.
More than 600,000 security personnel are being deployed nationwide and around half the estimated 70,000 polling stations have been declared at risk of attack, many of them in parts of Balochistan province and the northwest.
A senior PPP official said party chairman Bilawal Bhutto Zardari -- the son of former prime minister Benazir Bhutto -- would not vote because of threats to his life.
On Friday, a candidate for the provincial assembly in Sindh was shot dead along with two supporters in Karachi.
May 11, 2013 - Updated 127 PKT