The main opposition party in Tanzania's semi-autonomous Zanzibar islands called for a boycott of a planned re-run of disputed presidential, parliamentary and municipal elections on the Indian Ocean archipelago on March 20.

Zanzibar's electoral body annulled a previous ballot on Oct. 25 on grounds of fraud. The opposition Civic United Front (CUF) says it won those elections and has warned of violence on the islands if the rescheduled ballot goes ahead.

Tanzania has been one of Africa's most politically stable nations but Zanzibar has been a hotbed of opposition to central government, with strong secessionist and Islamist voices. Polls on the islands are usually closely fought and often disputed.

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"The Civic United Front (CUF) will not take part in the vote re-run ... because it is illegal. A lawful election has already been conducted (in October)," the party said late on Thursday.

"We urge all the people of Zanzibar ... to boycott this unlawful re-run election," it said in a statement.

Zanzibar's President Ali Mohamed Shein of the ruling CCM party stood in the previous polls against his main challenger, Seif Sharrif Hamad of the CUF party, who has lost four elections since 1995 by narrow margins.

There were other presidential candidates from smaller parties, but elections in Zanzibar have always been a tight two-horse race between the CCM, which has ruled mainland Tanzania for more than five decades, and the CUF.

The CUF has warned that violence could erupt on the islands if fresh elections are held. It says a dialogue to form a government of all parties under its leadership should be held rather than a new vote.