At least 17 opposition Civic United Front supporters have been injured, five sustaining bullet wounds, in clashes with police on Zanzibar.
Tensions has escalated ahead of polls later this month
Witnesses said police used tear gas and stun grenades to disperse hundreds of people prevented from attending a CUF rally on Sunday.
CUF spokesman Salim Bimani called on Zanzibar's police chief to resign.
Tensions have risen as Tanzania's semi-autonomous archipelago of Zanzibar prepares for elections on 30 October.
"The CUF supporters threw stones at the police after refusing orders to disperse. The police had to respond," Ramadan Kinyongo deputy director of criminal investigations at Zanzibar's police said, AFP reports.
Hospital officials told the BBC that five people were receiving treatment for bullet wounds.
The latest violence follows repeated clashes between supporters of the CUF and Zanzibar's governing Chama Cha Mapinduzi party.
The opposition accuses the governing Chama Cha Mapinduzi of planning to rig the ballot.
The CUF maintains that it has been cheated of victory in Zanzibar - its main power base - in the past two Tanzanian elections, in 2000 and 1995.