A suicide blast at a mosque inside a police headquarters in Pakistan that killed more than 90 people was a targeted revenge attack, a police chief said Tuesday.

Between 300 and 400 policemen had gathered for afternoon prayers at the compound's mosque Monday in the provincial capital Peshawar when an entire wall and most of the roof were blown out, showering rubble on officers.

"We are on the frontline taking action against militants and that is why we were targeted," city police chief Muhammad Ijaz Khan told AFP.

"The purpose was to demoralise us as a force."

The head of Khyber Pakhtunkhwa province police force, Moazzam Jah Ansari, told reporters that a suicide bomber had entered the mosque as a guest, carrying 10-12 kilograms (about 22-26 pounds) of "explosive material in bits and pieces".

He added that a militant group that was on-and-off affiliated with the Pakistani Taliban could be behind the attack.

Authorities are investigating how a major security breach could happen in one of the most tightly controlled areas of the city, housing intelligence and counter-terrorism bureaus, and next door to the regional secretariat.