Opposition and rights groups allege massive rigging before Monday’s election.
Two civilian passers-by were killed and eight security personnel wounded, two of them seriously, Major-General Athar Abbas, the Pakistan army’s spokesman, said.
Elsewhere on Saturday, officials said they had foiled an attack planned for polling day.
Supporters of Hussain were going into his office after a rally when the bomber launched the attack, witnesses said.
He said: “The motive of the blast may or may not be linked with the election.
“But the high death toll does point to a planned attack amid the election campaign.”
The blast comes as politicians in Pakistan launched a final push for votes before the midnight (1900 GMT) deadline after which all rallies are banned.