An overnight curfew has been imposed in the Indian Ocean nation of Comoros, the interior ministry said, after violent protests against President Azali Assoumani's re-election.

Assoumani won a fourth five-year term after the country's electoral body on Tuesday declared him the winner of Sunday's election against five opponents.

The army fired tear gas to disperse protesters in Moroni on Wednesday, and protesters were still on the streets in the north of the capital in the early hours of Thursday.

The interior ministry announced the curfew on Wednesday.

A government spokesperson blamed the protests on supporters of losing candidates.

"These are things that happen here and elsewhere, especially when we are beaten and we contest the results," Comorian government spokesperson Houmed Msaidie told Reuters.

He said several demonstrators had been arrested.

At least four people told Reuters on Thursday that they had trouble connecting to the internet to use social media platforms because of service disruptions.

Assoumani's opponents have said the election was tainted by voter fraud, alleging instances of ballot stuffing and of voting ending before the official closing time. The government has denied those accusations.

Assoumani first came to power through a coup in 1999. He stepped down in 2002 and then won elections 14 years later.

Constitutional reforms in 2018 removed a requirement that the presidency rotate among its three main islands every five years, allowing Assoumani to seek re-election in 2019.

He won 62.97% of the vote in the latest election, according to the national electoral commission.

With a population of about 800 000 people, Comoros has experienced around 20 coups or attempted coups since winning independence from France in 1975 and is a source of irregular migration to the nearby French island of Mayotte.

The United Nations High Commissioner for Human Rights called for calm and appealed authorities to practise restraint in the wake of protests.