Zanzibar's election commission chief says the results of the island archipelago's presidential election have been nullified because of alleged irregularities.

Jecha Salim announced Wednesday that the decision was taken because of several issues with the voting process. He did not say when another election will take place.

Tensions have been rising in Zanzibar since the Civic United Front, the main opposition party, claimed Monday that its candidate had won, days before the official results. The Civic United Front's candidate for president, Maalim Seif Hamad, asserted he won the election with more than 52 percent of the votes. His statement did not say how he had arrived at the result.

The Zanzibar island archipelago is a semi-autonomous part of the United Republic of Tanzania, whose presidential election results are expected Thursday.