The ruling CCM party’s presidential candidate in Zanzibar, Hussein Mwinyi, was declared the winner with 76% of the vote.
His rival Maalim Seif Sharif was arrested while trying to vote on Tuesday before being released hours later.
Mr Mwinyi is the son of a former president from the same party.
Zanzibar has a history of contested polls, including in 2015 when they were annulled for not being free and fair. The opposition boycotted the re-run and the CCM party’s candidate was declared the winner.
Ali Mohammed Shein is now stepping down after serving two five-year terms in office.
