Comoros will vote in an election on Sunday that is expected to deliver a fourth term to President Azali Assoumani, a former military officer whose opponents accuse him of muzzling dissent in the Indian Ocean archipelago nation.

Assoumani, who has served as the chair of the African Union for the past year, will face five competitors. Other opposition leaders have called for a boycott, accusing the electoral commission of favouring the ruling party.

The electoral commission has denied this and said the election will be transparent.

Regional observer missions, including from the African Union, said the last election in 2019 was riddled with irregularities and lacked credibility.

The earlier vote followed constitutional reforms that removed a requirement that the presidency rotate among the country's three main islands every five years, and thus allowed Assoumani to seek re-election.

The changes sparked months of sometimes violent protests in the nation of fewer than 1-million people, which has experienced around 20 coups or attempted coups since winning independence from France in 1975 and is a major source of irregular migration to the nearby French island of Mayotte.

Under the new system, Assoumani, who first took power in a coup in 1999 before stepping down in 2002 and then winning election 14 years later, would be required to step down in 2029.

Since 2019, Assoumani's government has cracked down on dissent, critics say. Former president Ahmed Abdallah Sambi was sentenced to life in prison in 2022 for high treason related to corruption allegations. Political protests have been repeatedly banned for security reasons.

"Democracy only exists in the lying discourses of Azali," said the main opposition leader, Mohamed Ali Soilihi, who lives in exile in France and has called for an election boycott.

Assoumani denies that anyone is prosecuted for political reasons and has vowed the election will go ahead successfully despite the boycott calls.

"Those who don't want the elections to take place have two options: stay at home or leave the country," he told reporters this week.

On the campaign trail, he has touted the construction of roads, hospitals and other infrastructure during his tenure.