A former French serviceman has been detained in Madagascar for allegedly taking part in a plot to destabilise the island, authorities say.

Prosecutor Nomenarinera Mihamintsoa Ramanantsoa said the French national had been charged with multiple offences including criminal conspiracy and plotting to sabotage infrastructure such as power lines and thermal plants.

Speaking in a video statement on Tuesday evening, he said other accomplices - including a Malagasy army officer - had also been charged for the alleged plot, set to be carried out on April 18.

Madagascar's foreign ministry has also expelled a French diplomat for alleged involvement in destabilising the country, which France has denied.

The French foreign ministry responded by summoning Madagascar's charge d'affaires in Paris, to "vigorously protest" over the expulsion of its official, rejecting "any accusations of destabilising the regime".

Its statement on Wednesday called the accusations "not only unfounded, but also incomprehensible given the consistent and concrete support provided by France".

Malagasy prosecutor Ramanantsoa on Tuesday said the suspects in the alleged plot had also been charged with spreading false information to disturb public order, and harbouring wanted individuals.

Authorities referred to a Whatsapp group chat called Revolution of the Brave Citizens as evidence of their alleged plans, said to include power cuts, attempting to incite security forces to rebel against the government, and mobilising young people to create unrest.

Last September, the Indian Ocean island was rocked by weeks of youth-led protests, which helped topple the country's then President Andry Rajoelina.

Col Michael Randrianirina led the coup and was sworn in as head of state in October 2025.

Some of the young people behind last year's protests have since fallen out with him, warning him not to "hijack the rebellion".

Young people were also among those detained on Tuesday.

Ramanantsoa said the young members of the group have been placed under judicial supervision, while the French former serviceman is currently being held in detention at a maximum security prison.

A former French colony, Madagascar has close political links to France and has a history of instability since independence.