Opposition parties in Madagascar have denounced what they say is an "institutional coup" after the prime minister, an ally of President Andry Rajoelina, was put in charge of the nation in the run-up to a presidential election.

In a letter to electoral authorities received on Tuesday, 10 of 13 presidential candidates said court rulings that handed the reins of the Indian Ocean island to Christian Ntsay were dictated by Rajoelina and his entourage to favour the president in the November election.

"The powers (that be)... carried out a real institutional coup with the aim of putting the Prime Minister in charge of the state during the presidential electoral period in order to manipulate the results for the benefit of their candidate," the signatories, including two former presidents, wrote. 

Voters in Madagascar, one of the poorest countries on the globe despite vast natural resources, head to the polls to elect a president on November 9.

Rajoelina, 49, resigned on Saturday, in line with the constitution, in order to run for re-election.

The President of the Senate was supposed to take over but declined to do so for "personal reasons", leaving the task to a "collegial government" headed by Ntsay.

The move was accepted by the country's top court -- which over the weekend also dismissed appeals to have Rajoelina's candidacy declared void over his dual French nationality -- sparking the anger of the opposition.

"The renunciation made by the President of the Senate is not provided for anywhere in the Constitution," the letter read.

The head of the Constitutional Court dismissed accusations of bias.

"The President of the Senate renounced the office. We cannot force him to take the reins of power," Florent Rakotoarisoa told AFP, adding the decision was lawful and aimed to "ensure the continuity of the State".

'Confidence is crumbling'

On Saturday the court had also rejected appeals filed by three opposition parties to have Rajoelina's name struck off the ballot as "unreceivable".

Media reports in June revealed that Rajoelina had been naturalised as French in 2014.

Under local law that meant he could lose Madagascan nationality and thus the ability to lead the country and run for office.

This version of the facts was vigorously disputed by the ruling party.

Rajoelina had not waited for the court ruling to announce his candidacy, holding a grand campaign launch at a stadium in the capital last week.

He first took power in 2009 on the back of a coup that ousted former president Marc Ravalomanana, who is also running in November.

After not contesting in the 2013 election due to international pressure, Rajoelina was voted back into power in 2018.

Eleonore Johasy, national coordinator for presidential candidate Auguste Paraina, said the Constitutional Court rulings were "taken at odd hours, so that there would be no chance to oppose them."

"Confidence is crumbling. All the shenanigans and manoeuvres do not promote trust in the different authorities," she told AFP.

-AFP