Violent clashes on Saturday between opposition supporters and Tanzanian police on the semi-autonomous islands of Zanzibar and Pemba left at least 11 people dead as police beefed up security on the mainland.
Hospital, police and journalists said 20 people were seriously injured, after police cracked down on supporters of the Civic United Front (CUF) who had called for mass demonstrations to press for fresh elections.
In Chake Chake, the main city in Pemba, a police officer was decapitated by machete, a local journalist said.
In Wate, in the north, three demonstrators were shot dead when police intervened to disperse a demonstration, a witness told AFP.
A hospital official said a fourth protester had died of his wounds, while an AFP journalist reported seeing four corpses.
In Zanzibar Town, the main town in Zanzibar, five people were shot dead by police and another man died from machete wounds, witnesses and hospital sources said.
Three police officers were also seriously wounded.
Police fired live rounds and sprayed tear gas at dozens of opposition supporters there, some of them armed with machetes and petrol bombs.
Calm was restored on the islands by midday.
In Dar es Salaam, meanwhile, police beefed up security and prevented people from holding rallies, after the opposition had called for protests on the mainland and the archipelago, where polls held in October and November were widely viewed as undemocratic.
The presidential and legislative elections handed victory to the long-ruling Chama Cha Mapinduzi party both on the mainland and Zanzibar.
The violence on Saturday on the islands came a day after police shot dead two unarmed civilians in Zanzibar, relatives and hospital sources said.
Witnesses said police opened fire on a small group of people who had emerged from Friday prayers in mosque near an opposition office in Zanzibar town -- ZANZIBAR (AFP)