ZANZIBAR Violent clashes on Saturday between opposition supporters and Tanzanian police on the semiautonomous 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.

Political tension has been high on the islands since October and November polls which the opposition says were rigged in favor of the ruling Chama Cha Mapinduzi (CCM or Revolutionary Party).

On Saturday, 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. Saturday's violence 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.

Fearful of violence spilling over onto the mainland, police beefed up security on Saturday in Dar es Salaam and prevented people from holding rallies, after the opposition had called for widespread protests.

On Friday, police arrested around 50 suspected opposition supporters calling for demonstrations, after CUF leader Ibrahim Lipumba was taken in and charged with unlawful assembly and disturbing the peace.

Witnesses said the opposition leader and his supporters were beaten during the roundup.

The government, meanwhile, said it had unearthed plans by the CUF to "wreak havoc" across the country.

Appearing in court, Lipumba and 15 others have pleaded not guilty to charges of unlawful assembly and destruction of property.

Lipumba, a former economics professor at the University of Dar es Salaam wore a bandage on his left arm, while several accused persons had head injuries with plasters and a few had bloodstains on their clothes.

Presidential and legislative elections late last year handed victory to the long-ruling Chama Cha Mapinduzi Party in Tanzania and its offshore islands.

While polling was relatively trouble-free on the mainland, on Zanzibar and Pemba it was described by the CUF and international election monitors as undemocratic.

In 1994, the Sultan of Zanzibar and Pemba was overthrown in an armed uprising following which the islands united with mainland Tanganyika to form the Republic of Tanzania, AFP reported.