The affected mosque is adjacent to a police headquarters, i.e. in an extra heavily guarded area in Peshawar.
Many supplicants are assumed to be police officers.
A total of between 300 and 400 people are believed to have been there.
Police officer Shahid Ali says that it exploded just as the imam had begun his sermon.
He ran out of the building as black smoke began to spread.
- People's screams still echo inside my head.
People were screaming for help, Ali told AFP.
Information on the number of injured has varied in various authority statements and media reports.
A director of rescue services tells AP that over 150 people have been injured.
The border with Afghanistan
Peshawar is close to the border with Afghanistan and is the capital of what used to be called self-governing tribal areas - areas that have long been fraught with conflict.
A Pakistani Taliban commander initially claimed responsibility for the attack.
But the group's spokesperson later distanced itself from the bombing, insisting that they did not target mosques and religious sites, without commenting on why the commander had undertaken the act.
Since the Taliban took power in Afghanistan, their counterparts in the neighboring country have committed more acts of violence, according to the authorities.
Last November, the Pakistani Taliban ended a formal ceasefire that had been between them and the Pakistani government.