(AFP) - Ten out of 12 opposition candidates in Madagascar on Tuesday called on voters to shun a presidential election scheduled for Thursday, amid concerns about the vote's regularity.

For weeks, the Indian Ocean island nation has been shaken by a fierce battle between President Andry Rajoelina, who is running for re-election, and most opposition leaders, who have complained about an "institutional coup" in favour of the incumbent.

"We reject Thursday's elections and we call on all Malagasy people to consider that this election does not exist," Hajo Andrianainarivelo, 56, told a press conference in Antananarivo, speaking on behalf of the 10 presidential hopefuls.

The move is likely to worsen political tensions that have been running high in the country for more than a month.

Since early October, the opposition grouping has led near daily, unauthorised protest marches in the capital that have on average drawn a few hundred supporters and been regularly dispersed by police.

Six presidential candidates had already told AFP on Monday they planned to call for a boycott of Thursday's vote.

Now, the call has been formally endorsed by all 10 members of the group.

"We appeal to everyone not to vote. Conditions for a transparent presidential election, accepted by all, have not been met," said Roland Ratsiraka, one of the protesting candidates.

"We do not want to participate in this fraud, it is a joke on Madagascar."

The opposition grouping initially counted 11 members, but one, Siteny Randrianasoloniaiko, later broke ranks and started campaigning -- something the others have refused to do in protest.

Another candidate has remained on the sidelines.

The country, which gained independence from France in 1960, has a long history of disputed elections.

- Electoral 'sabotage' -

The latest crisis erupted in September after Rajoelina resigned in line with the constitution in order to run for re-election.

The president of the Senate was supposed to take over but declined for "personal reasons", leaving the task to a "collegial government" headed by the prime minister, an ally of Rajoelina.

The move was accepted by the Constitutional Court, which also dismissed appeals to have Rajoelina's candidacy declared void over his dual French nationality, sparking opposition anger.

Detractors believe that under local laws the president should have lost Madagascan nationality and with it the ability to lead the country because of the naturalisation, which was revealed by the media in June.

The opposition has also denounced electoral irregularities, alleging discrepancies in the voters' roll and the existence of "thousands of fictitious polling stations".

Rajoelina's government has repeatedly condemned opposition protests as moved by a "desire to overthrow power" and to "sabotage the electoral process", accusing his challengers of "threatening the stability of the country".

Last week, the head of the lower house of parliament, who leads a mediation group to find a way out of the crisis, called for the suspension of the elections, saying the situation did not allow for a free and credible vote.

But a spokeswoman for Rajoelina called the request a "far-fetched idea".

The outgoing president held his last campaign rally in Antananarivo on Sunday in front of a fervent crowd of several thousand people donning T-shirts bearing his image.

"I'm going to win, that's for sure, and in the first round," he told AFP in an interview at the weekend.

Rajoelina, 49, became Africa's youngest head of state in 2009 when he took power on the back of a coup.

After not running in the 2013 election due to international pressure, he was voted back into power in 2018 and has since held the reins in a country that remains among the poorest in the world despite vast natural resources.

Voters were initially due to head to the polls on November 9, but the top court ordered a postponement after a presidential candidate was injured during a demonstration.

The date for a potential second round vote on December 20 was kept unchanged.

"We want an election," said Andrianainarivelo but "we will continue to protest until there is an election accepted by all."

The European Union, the United States and other members of the international community have expressed "deep concern" at the political tensions and denounced the excessive use of force against the opposition.

© Agence France-Presse