Herat police chief's spokesman Abdul Ahad Walizada has confirmed an explosion ripped through Nabi Akram Mosque in PD7 in Herat city.
Walizada said at least one person was killed and seven wounded in the explosion but said the casualty toll could rise.
He said victims had been taken to local hospitals.
According to him, two attackers approached the mosque on a motorcycle. When police stopped them the one detonated his explosives. Police gunned down the second attacker.
Walizada said the explosion tore through the mosque during mid-day prayers - shortly after 1pm.
No details have been released on the type of explosion but Walizada said police are investigating.
Daesh has claimed responsibility for the incident.