"The victims of the attack, unfortunately, are all religious scholars who gathered to commemorate the birthday of Prophet Muhammad," said Basir Mujahid, spokesman for the Kabul police chief, according to the Associated Press.
Police security had not been requested for the event, and the bomber entered the hall easily, Mujahid said, according to the AP.
There has been no claim of responsibility for the attack. The Taliban denied involvement.
Afghanistan's President Ashraf Ghani announced that a national day of mourning would take place Wednesday.