The bomber targeted a paramilitary convoy as it passed through a security checkpost in a bazaar in Alpuri town in Shangla, a district neighbouring Swat valley and the target of a recent anti-Taliban military offensive.
"Thirty-two people have been killed -- 28 of them are civilians and four are security personnel," Major Mushtaq Khan, a spokesman at the military-run Swat Media Centre, told AFP.
"Forty-six people are injured -- six of them are troops. Five of the troops are in a serious condition. It was a suicide blast. The attacker was on foot."
Fazllulah Khan, a Shangla member of parliament, confirmed the death toll, but said on the private Geo television channel that his information suggested that the suicide bomber rammed an explosives-packed car into the convoy.
"The target was a security convoy near an army checkpost. This is a crowded bazaar and a lot of people were present at that time," he said.
Swat valley was the target of a punishing military offensive launched in April this year, with the army claiming to have now flushed Taliban rebels out of the one-time tourist paradise.
Fighting also spilled into Shangla, where Taliban militants had advanced in a bid to impose a harsh brand of Islamic law across the northwest.
Alpuri was known to be a stronghold of fugitive Swat Taliban commander Maulana Fazlullah, who remains at large, raising concerns that the Swat Taliban are regrouping in the northwest Pakistan´s rugged mountains.
There was no immediate claim of responsibility for the blast, but the Pakistani Taliban have vowed to avenge both the Swat military offensive and the death of their leader Baitullah Mehsud in a US missile strike in August.
Pakistan has been hit by a wave of attacks blamed on Islamist extremists in the past week, with at least 52 civilians killed on Friday when a suicide bomber rammed his car into a market in the northwest capital Peshawar.
Then on Saturday, suspected Taliban-linked gunmen staged an audacious day-time raid on the army headquarters near Islamabad, shooting their way into a building and barricading themselves inside with 42 hostages.
In total, eight militants, 11 soldiers and three hostages were killed in the crisis that unfolded at the heart of the military establishment in the garrison city of Rawalpindi, which ended with a commando raid Sunday.
The military are now readying for a full-on ground assault into the Taliban strongholds in the northwest tribal belt neighbouring Afghanistan.
Related story
Blast at market in northwest Pakistan kills at least 25
