The seminary targeted by the blast is located at Gulshan Colony in Peshawar, the capital of Khyber-Pakhtunkhwa province.
The bomber targeted the Madrassa Hussainia at a time when about 200 men had gathered for the weekly Friday prayers.
The men were listening to the sermon by the prayer leader when the bomber entered the madrassa from its rear. The bomber opened fire before detonating his explosive vest.
At least six persons were killed instantly and officials said the death toll could rise. At least four injured men were in a serious condition, they said.
The walls and ceiling of the seminary were pitted by ball bearings packed into the bomber's explosive vest. The building's windows and doors were blown out by the blast.