“The blast took place during the evening prayers inside the mosque in Police District 17, leaving 21 dead and 33 others injured at the spot,” Khalid Zadran, the Kabul police spokesman, told reporters.
Witnesses heard a powerful explosion which shattered windowpanes of the nearby buildings, reports Xinhua news agency.
No one has so far claimed responsibility for the deadly incident.
The blast came a few days after a prominent pro-Taliban cleric was killed in a suicide bomb blast in Kabul.
The Islamic State terror group had claimed responsibility for the earlier attack.