Three people have been killed at a mosque in north-west Tanzania in what police have described as a "terrorist-style attack".
More than a dozen attackers carrying machetes and axes struck during evening prayers at the mosque in Mwanza district on Thursday, police said.
The police have made three arrests.
The BBC's Sammy Awami in Tanzania says there are fears over the growing threat of Islamist militancy in the country, which has so far avoided such attacks.
No group has said it was behind the mosque killings.
The mosque's imam is among the dead.
According to eyewitnesses, the masked attackers, who were carrying black flags, switched off the lights, before asking worshippers: "Why are you praying while our fellow Muslims are unfairly held by the police?"
Police have confirmed that the attackers were unhappy about recent arrests of Muslims in the area.