ZANZIBAR, Tanzania (AP) — The political future of Zanzibar, a semiautonomous archipelago off the coast of Tanzania, was thrown into doubt Wednesday when its election commission chief nullified results of an election that the opposition claims it won.

The results were scrapped because of several issues, Jecha Salim announced without saying when new elections will take place.

The U.S. Embassy said in a statement that it is "gravely alarmed" and called for a reversal of the annulment.

"This action halted an orderly and peaceful election, as evaluated by observer missions from the U.S. Embassy, European Union, Commonwealth, and Southern Africa Development Community, and a tabulation process nearing completion," the statement said. "We call for this announcement to be recalled, and urge all parties to maintain a commitment to a transparent and peaceful democratic process."

Zanzibar's main opposition party, the Civic United Front, expressed "outrage" at the election annulment. "This is a deliberate effort to thwart democracy in Zanzibar and cause chaos in the country," said the party in a statement. By nullifying the election results the electoral commission "has plunged the United Republic of Tanzania into a crisis," said the statement.

Tanzania was formed when Zanzibar and its northern island, Pemba, united with mainland Tanganyika in April 1964, three months after the islands' black majority overthrew the traditional Arab rulers. But for years, many have felt that Zanzibar, and its ancient capital, Stone Town, have been relegated to second-class status.

Zanzibar is semi-autonomous and elects its local government, including a president, but many on the islands complain that the mainland Tanzanian government dictates policies, especially on economic issues.

On Monday, Zanzibar's opposition presidential candidate, Maalim Seif Hamad of the Civic United Front, claimed that he had won, days before the official results were to be released. Armed police were then deployed in Stone Town to break up crowds of opposition supporters who had gathered to celebrate. Helmeted police made some arrests.

The Civic United Front has tried for years to win control of Zanzibar's local government from the ruling Chama Cha Mapinduzi (Revolutionary Party), narrowly losing the last election in 2010

On mainland Tanzania, the results for the general and presidential elections are expected Thursday. It is the most tightly contested race since Tanganyika became independent from Britain in 1961.

In the race for the president of all Tanzania, the ruling party's candidate is Works Minister John Magufuli. He is challenged by former Prime Minister Edward Lowassa, who used to be a high-ranking member of the ruling party until he defected to the opposition after failing to win the ruling party's presidential nomination.

Lowassa told reporters in Dar es Salaam, Tanzania's largest city, that he would challenge the results being announced by the election commission, which he accused of falsifying some figures to favor Magufuli. Results from some constituencies that have been announced by the election commission do not tally with the numbers of votes that were cast in those places, he said.

Damian Lubuva, the head of Tanzania's election commission, denied the charges while issuing partial results Wednesday.