Swat police Chief Idrees Khan said items found at the scene of the attack in Mingora suggested it may have been a suicide bombing, though an investigation would be needed to confirm it.
Witness Rahim Gul said the blast occurred as two vehicles carrying Pakistani security forces passed through the busy Nishat intersection, which is surrounded by small shops and stalls. But the security forces' vehicles appeared to be undamaged, Gul said.
Television footage of the scene showed several cars gutted by fire or with blown-out windows. Heavily armed security forces were guarding the area.
Mingora is the capital of Swat, a mountainous region near the border with Afghanistan where Pakistani military forces last year launched a major offensive to remove Taliban militants who had taken control. The militants were driven out, but violence including bomb attacks has continued periodically.
Khan said three people were killed and at least eight others who were wounded were rushed to a hospital.
Another witness, Shiraz Khan, said people could be heard crying for help after the blast.