The explosion occurred outside the office of Awami National Party (ANP) leader Fateh Khan in Dagar area of Buner district of Khyber-Pakhtunkhwa province, about 100 km from Islamabad.
The blast left five people dead and several others injured, police said.
The identity of those killed could not immediately be ascertained.
The ANP leader's office was located a short distance from a government-run medical centre.
No group claimed responsibility for the attack.
Security forces cordoned off the area and launched a search operation.
Scores of leaders and workers of the ANP have been killed in attacks by the banned Pakistani Taliban in the past four years.