Zanzibar's opposition chief called for calm Monday, having previously threatened to lead protests if results of annulled elections were not released on the semi-autonomous Tanzanian islands.

Troops patrolled the streets of the capital of the Indian Ocean archipelago, after homemade bombs exploded in Zanzibar town over the weekend, an AFP reporter said.

Zanzibar's electoral commission ruled last week that the October 25 vote on the islands — where the 500,000 registered voters also cast ballots for Tanzania's national president — must be carried out again, citing "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 threatened to call for protests if the situation was not resolved by Monday, saying he would not concede victory to his main rival, incumbent president Ali Mohamed Shien of the long-running Chama Cha Mapinduzi (CCM).

But on Monday Hamad urged supporters to allow a political deal to be struck.

"I ask CUF fans and Zanzibaris to remain calm and observe peace — diplomatic efforts have been going on well," Hamad said. "Let us be patient, as the international community is also helping to have Zanzibar's political crisis resolved."

Outgoing Tanzanian President Jakaya Kikwete said in a statement on Sunday he had been "working tirelessly and consulting widely over the last few days to find an amicable and peaceful resolution to the situation in Zanzibar."

African and other international observers in Tanzania have said they are deeply concerned at Zanzibar's annulment, urging leaders to "cast aside their differences" to ensure peace.

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

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

"Some people including politicians are misleading Zanzibaris by saying the islands would have no legitimate president after November 2, 2015," the statement read. "The president is here to stay until next elections."

The twin blasts on Saturday caused no casualties but the explosions sparked concern on the islands, whose economy is dependent on foreign tourists.

Late Friday, explosives experts also detonated a homemade device in the heart of the historic Stone Town district, a UNESCO-listed area.