An opposition party in the island archipelago of Zanzibar has declared victory in a presidential election here even as official tallying continues, raising tensions as mainland Tanzania awaits official poll results.

Maalim Seif Hamad of the Civic United Front party announced he had won the Zanzibar presidential election, saying he had ousted his rival, Ali Mohamed Shein of Tanzania's ruling party.

He urged Tanzanian President Jakaya Kikwete "to facilitate a smooth transition and not to allow Zanzibar to descend into chaos."

Police fired tear gas to disperse Hamad's supporters who had gathered to celebrate here and on the island of Pemba.

Although part of Tanzania, Zanzibar is run by a semi-autonomous government with its own president.

Full results from Tanzania's largely peaceful presidential election are expected on Thursday.