Television footage showed a blast crater 3 feet deep, destroyed vehicles and pieces of debris scattered across a large area.
Officials said many people were trapped under the rubble of two collapsed buildings in a nearby market. Civilians dug frantically with their hands in hopes of finding survivors.
Nasirulmulk Bangash, a top police official in the area, said the vehicle carried at least 330 pounds of explosives - an amount he called "unprecedented" - and was apparently en route to Peshawar, the capital of the North West Frontier Province.
He said the large quantity of explosives indicated the attack was aimed at a more important target than the small checkpoint, but might have been tripped up by high security on election day.
Bangash did not speculate on what the intended target might have been.
Peshawar police chief Muhammad Suleman said late Saturday that 30 people died, including five police officers and several who succumbed to their injuries in hospital or were found lifeless among the rubble.
He said the toll was likely to rise as rescue teams continue to dig.
Sher Zaman, 15, told The Associated Press from a hospital bed that he was selling fruit from a cart at the market when he heard a large explosion and was knocked down when something hit him in the chest. He said residents quickly gathered and helped transport casualties to hospitals.
No one immediately claimed responsibility for Saturday's blast.
In recent weeks, however, the Pakistani Taliban have said they were to blame for a string of suicide bombings they called revenge for military offensives in the northwest region, which borders Afghanistan. One attack killed nearly 70 people at a major weapons factory.
Separately, the military said, residents of the Matta area, about 106 miles north of Peshawar, fought back against militants in the area, launching an attack Friday night to prevent the anticipated kidnapping of a village elder accused of supporting the government. Six of the would-be kidnappers were killed.
The militants later returned en masse, and ensuing clashes left 15 residents and three more militants dead, along with scores of people injured, the military said in a statement.
Pakistan has struggled to contain rising militancy in its borders, and the fledgling government has tried both peace talks and military operations to stop the insurgents.
It's an effort watched closely by the U.S., which says that militants have safe havens in Pakistan's northwest from which they plan attacks in neighboring Afghanistan.
A recent U.S.-led ground cross-border assault on a Pakistani tribal region, said to have killed at least 15 people, prompted protests from the government and angered many Pakistanis.
Asif Ali Zardari, widower of slain ex-Prime Minister Benazir Bhutto, won the presidency. He has vowed to be tough on militancy.