A suicide bomber stormed into the largest Shiite Muslim mosque in Afghanistan's Herat province, killing at least 20 and wounding dozens more, said the provincial governor's spokesman Jalani Farhad.
However Mehdi Hadid, a politician from Herat who went to the site soon after the explosion, told The Associated Press that the scene was one of horrific carnage.
He estimated at least 100 dead and wounded were scattered throughout the mosque.
The attack took place in the middle of evening prayers when the mosque was packed with about 300 worshippers.
At Herat Main Hospital, Dr Mohammed Rafique Shehrzai confirmed 20 bodies were brought to the hospital in Herat city, the capital of western Herat province, soon after the blast.
Shiites are a minority in Afghanistan and have been threatened by the affiliate of the Islamic State group that operates in the country's east.
No-one immediately claimed responsibility for the attack on the Jawadia Mosque, the largest Shiite Muslim mosque in Herat.
Mr Hadid said he rushed to the mosque after hearing about the explosion.
He was told the attacker first fired on the private guards outside the mosque before entering.
When inside, he fired on the praying worshippers until his rifle jammed then he blew himself up.