Zanzibar's electoral commission on Wednesday annulled elections on the semi-autonomous Tanzanian archipelago citing "violations of electoral law".
Zanzibar Electoral Commission (ZEC) chairman Jecha Salim Jecha, in a statement broadcast on public television, said the polls on the Indian Ocean archipelago were "nullified" and must be carried out again.
Jecha alleged violations including double-voting and cheating.
On Monday, the main opposition presidential candidate declared himself the winner before the results were officially announced.
Seif Sharif Hamad from the opposition Civic United Front (CUF), the vice-president in the previous unity government, told reporters he had won Sunday's polls, claiming to have seen official documents of final voting figures.
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Security forces on Tuesday surrounded the islands' main tallying and results centre, after Hamad repeated warnings that he, "will not concede defeat if robbed of my victory."
Troops left the centre later Tuesday, but an AFP reporter in Zanzibar's capital said shots were heard overnight, and that the streets were largely empty on Wednesday, with many shops closed and people saying they were fearful of going out.
Police on the Indian Ocean islands fired tear gas to break up crowds, while foreign embassies warned visitors to the popular tourist destination to avoid large crowds.
Zanzibar has experienced sectarian and political tensions in recent years -- including several grenade explosions -- with the unrest affecting the islands' key tourist industry.
There have also been wider tensions around Zanzibar's union with the mainland, with some opposition political parties wanting to break ties and return to independence.
The long ruling CCM has vowed to maintain the status quo, but the CUF promises to campaign for full autonomy if it wins.
In mainland Tanzania, votes are being counted following Sunday's presidential, general and local elections.
Zanzibaris also voted for the national president.