Zanzibar has formally annulled October's elections meaning a rerun appears more likely, despite continued negotiations to strike a political deal on Tanzania's semi-autonomous islands.

Despite fierce opposition criticism, the government gazette on Wednesday formalised the nullification of the October 25 polls, cancelled after the Zanzibar Electoral Commission (ZEC) cited "violations of electoral law".

The annulment came after a key candidate, Seif Sharif Hamad of the opposition Civic United Front (CUF), declared himself the winner before the results were officially announced.

Hamad and incumbent President Ali Mohamed Shein from ruling party Chama Cha Mapinduzi (CCM) have met at least twice in a bid to resolve the crisis, media reports say.

A government statement has said Shein would remain in charge until fresh elections.

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"The date for fresh elections will be announced later," Zanzibar information ministry official Rafi Haji told AFP.

Zanzibar's 500,000 registered voters also cast ballots for Tanzania's national president, and despite the cancellation on the islands, new Tanzanian President John Magufuli has been sworn into office.

He took power along with the east African nation's first female vice-president, Samia Suluhu Hassan, who comes from Zanzibar.

Magufuli's win in the October 25 poll with over 58 percent of votes cemented the long-running CCM's firm grip on power.

But on Zanzibar, African and other international observers have said they were deeply concerned and have urged leaders to "cast aside their differences" to ensure peace.

Zanzibar has experienced sectarian and political tensions in recent years, including several grenade explosions.

Several homemade bombs have detonated in Zanzibar town since the polls -- including in the heart of the historic Stone Town district, a UNESCO-listed area popular with tourists -- although there have been no injuries.

There have also been wider tensions around Zanzibar's union with the mainland, with some opposition parties wanting to break ties and return to the independence it briefly enjoyed in early 1964 before merging with Tanganyika.

The CUF promised to campaign for full autonomy if it wins, while the CCM has vowed to maintain the status quo.