The announcement in the centre of Zanzibar's Stone Town capital brought thousands of pro-government supporters dressed in green and yellow party colours pouring into its narrow alleyways hooting horns, waving flags, dancing and celebrating.
"I announce officially that Amani Abeid Karume is re-elected," Electoral Commission chairman Masauni Yusuf Masauni said, unveiling the third consecutive ruling party win on the former slave-trading islands long seen by the opposition as a hotbed of electoral fraud.
The main opposition Civic United Front (CUF) dismissed the victory as "cooked up".