A group of gangsters on Wednesday night stormed a mosque in Tanzania's northern region of Mwanza, killing three people, including the leader of the mosque, police have said.

Ahmed Msangi, the Mwanza regional police commander, said the trio was hacked to death by sharp objects believed to be machetes.

A 13-year-old boy was wounded as he tried to get out of the mosque using the women's entrance. He has been rushed to the Nyamagana district hospital.

Two survivors of the attack said the gangsters displayed a black flag with white words reading "Islamic State".

"They were about 15, and had covered their faces with masks when they stormed into the mosque," said the survivors.

"The attackers switched off lights in the mosque before they started ordering children to walk out through a rear door usually used by women," said one of the survivors, Abubakar Makabwe.

"Before they began slashing our colleagues, they asked us why we were in the mosque while our fellow believers were in police custody," added Makabwe.

Another survivor, Ridhwan Abdallah, said it appeared to be a woman who ordered the killings.

"It was a woman, according to the voice, who ordered that the leader of mosque should be the first to be killed," said Abdallah.

Police identified the leader of the Rahman mosque as Feruz Ismail, 27, and the two others killed as Mbwana Rajab, 40, and Khamisi Mponda, 28.

Msangi said the killings bore the hallmark of Somalia-based Islamist group, Al-Shabaab, which has staged waves of attacks in Somalia and neighbouring Kenya.

Police say they are holding a number of suspects in connection with the incident.

It is not immediately clear whether the attack is linked to the Islamic State (IS) militant group.