"Five policemen and two paramilitary soldiers were killed in the bomb blast," Gran Ullah, the local police station chief, said.
"A pedestrian also died in the blast," he added.
Al Jazeera's Kamal Hyder, reporting from Islamabad, said: "This is a very unusual and deadly attack. A car was discovered on the side of a road with a dead body of a man inside, the police then rushed to investigate.
"We are told that as they were closing in on the vehicle, it was detonated by what appears to be remote-controlled device. Police are still investigating the incident," he said.
Violence in Pakistan has surged in recent months amid a wave of attacks blamed on Taliban fighters.
There have been regular battles with armed fighters in the north-west, particularly in the Swat valley and in the tribal region of Bajaur.