Police say the attacker pretended his car had broken down
Rescuers in Pakistan have been working into the night to search for survivors in the rubble left by a car bomb at a school being used as a polling station.
At least 33 people died in the attack during a by-election in the north-western district of Buner, on the edge of the restive Swat valley.
The bomb destroyed the school, trapping people under the rubble. Two police were among the dead and 15 hurt.
The Taleban say it was in response to an attack on their fighters.
Six militants were shot dead a few months ago when the local residents turned against the Taleban, in what correspondents say was one of the first incidents of villagers turning against the Taleban in the region.
Police said the school was being used as a polling station for a by-election for a parliamentary seat. Voting was suspended after the attack.
"The suicide attacker pulled his car outside the polling station, and asked people to push the vehicle, saying that it had broken down," North West Frontier information minister Mian Iftikhar Hussain told the Associated Press.
"His purpose was to gather the maximum people around the car. The moment people started pushing the car, he blew it up."
Security forces have been fighting militants in the Swat valley, formerly a popular tourist destination, since last year.