(AFP) - The Comoros declared a curfew on Wednesday after security forces clashed with protesters angered by the re-election of President Azali Assoumani in a vote opposition leaders denounced as fraudulent.

During the day, demonstrators ransacked a former minister's house and set it on fire, as others tried to block roads in the capital. Police responded with tear gas and arrests, AFP reporters saw.

In the evening, citing "public necessity", the government ordered a night curfew.

The measure was to start at ten pm (1900 GMT) and last until six am on Thursday nationwide, except for Moroni where it kicked in a few hours earlier, the interior ministry said.

Election officials said on Tuesday that Assoumani had won 62.97 percent of the vote in Sunday's ballot.

But the five opposition challengers have cried foul, alleging ballot-stuffing and inconsistent results.

"Incontestably these ballots of Sunday January 14, 2024 are invalid. We denounce them and demand their pure and simple annulment," the candidates said in a joint statement.

Furniture and burning tyres were scattered across several roads of Moroni and the Indian Ocean archipelago's largest street market lay deserted in the morning.

Tear gas was fired well into the afternoon and smoke billowed over the city as police and the army tried to clear roads blockaded by demonstrators.

In Geneva, the United Nations appealed for "all to exercise restraint".

"As post-electoral tensions mount, it is paramount that the authorities ensure a safe environment, where all Comorans, including members of the political opposition, can freely express their views and exercise their right to peaceful assembly," UN rights chief Volker Turk said.

He urged the authorities to release those arbitrarily detained, to investigate rights violations in the pre-election period and bring perpetrators to justice.

Government spokesman Houmed Msaidie, speaking to AFP, accused the opposition of organising the protests.

"There have been arrests, but I can't give you the figure for the moment. It's totally normal when there are people out there who want to disturb public order," Msaidie said.

There have been no reports of deadly violence, but the Comoros -- a three-island chain with a population of about 870,000 -- is politically volatile and has seen 20 coups or attempted coups since independence in 1975.

Official results released Tuesday showed Assoumani, a former coup leader turned civilian president, won re-election in the first round.

- 'Flagrant fraud' -

But official turnout was unexpectedly low at 16 percent and large discrepancies in the number of votes reportedly cast for the presidential and regional governor races raised doubts about its regularity.

Opposition candidates said they were "horrified" to note that the official results implied improbably that more than two-thirds of people voted to elect island governors but failed to cast a presidential ballot in a parallel vote in the same polling stations.

"It's impossible," they said, adding this called into question the veracity of the official results.

"There is nothing to say: a flagrant fraud has been committed."

Police, gendarmes and armed soldiers were deployed in large numbers as the day began.

In the working-class Coulee district in the north of the city, groups of youths threw stones at the troops, but many residents were preparing to flee.

"Everyone is gone. I'm going too. I was tear-gassed," said Amina, a stallholder in the normally bustling Volo-Volo market, now just rows of empty wooden stands.

A former army chief-of-staff, Colonel Assoumani initially came to power in a coup in 1999, before handing over to civilians in 2006.

He returned to politics and won re-election in 2016 in a vote marred by violence and allegations of irregularities.

He has since been accused of creeping authoritarianism. His arch-rival ex-president Ahmed Abdallah Sambi was given a life sentence for high treason for allegedly selling passports.

During this year's campaign, Assoumani hailed his government's construction of roads and hospitals.

But in a country where 45 percent of the population live below the poverty line, plagued by electricity cuts and water shortages, he has faced popular criticism.

© Agence France-Presse