The bomb exploded just as senior ANP leader Bashir Jan arrived in Mominabad area to address an election meeting.
Jan, who has been targeted by the Taliban several times, escaped unhurt though his car was damaged by the blast.
Police officials and ANP leaders said 11 people were killed and more than 40 injured. Six children were among the wounded.
The banned Tehrik-e-Taliban Pakistan claimed responsibility for the attack. Taliban spokesman Ihsanullah Ihsan phoned media groups and said his group would continue to target the ANP, Muttahida Qaumi Movement and Pakistan People's Party for their secular and liberal policies.
More From This Section
"The ANP has tried to introduce infidel views in Pakistan. They will pay the price for this," Ihsan said.
Over the past few days, the Taliban have bombed two election offices of the MQM in Karachi. Ten people were killed and dozens injured in these attacks.
The target of tonight's blast was Bashir Jan, the secretary general for ANP's Sindh chapter who had earlier in the day escaped a grenade attack.
Police said the blast was apparently caused by a bomb planted in a motorcycle. Ball bearings were found at the site where the blast created a crater.
Doctors at Abbasi Shaheed Hospital said they had received around 20 injured people and the condition of some of them was critical.
Faisal Abdi, a rescue official, said the intensity of the blast was so great that it damaged parked vehicles and motorcycles and its sound was heard over a wide area.
"The Election Commission of Pakistan, law enforcement agencies, the IG police and DG Rangers have completely failed to provide us security despite our reminders to them. They are responsible for all those who have been killed or injured today," an emotional Syed said.