The blast happened in Dera Ismail Khan, where Shiites were holding a procession for the religious observance of Ashura, Geo News reported.
Police said it appeared a remotely detonated bomb had been planted in a bicycle repair shop, which was closed at the time, Dawn News said.
While it's become common for bombers to use cellphones as detonators, the federal government had ordered cell service suspended in major cities to prevent such attacks during Ashura, Dawn said. Bomb technicians were scouring the scene to see how the bomb was detonated, the report said.
A day earlier in the same city, a bomb attack at a religious parade killed four adults and four children, Geo said.
A Pakistani militant group associated with the Sunni Muslim Taliban in nearby Afghanistan claimed responsibility for Saturday's blast.