Madagascar has detained a former French serviceman accused of taking part in a plot to destabilise the Indian Ocean island, including by inciting security forces to mutiny and sabotaging infrastructure to cause nationwide blackouts, authorities said.

Separately, Madagascar's foreign ministry said on Tuesday it had declared an agent at the French embassy in Antananarivo persona non grata over acts deemed incompatible with diplomatic status and linked to the destabilisation investigation.

Madagascar, one of the world's poorest countries, is a former French colony which retains close political links to France and has a history of instability in recent decades.

The country's military ruler, President Michael Randrianirina, seized power ​in October last year, after a wave of youth-led protests against his ​predecessor, Andry Rajoelina.

The French embassy did not immediately respond to requests for comment.

SUSPECTS ACCUSED OF PLOTTING DISRUPTION, REBELLION
Deputy Prosecutor Nomenarinera Mihamintsoa Ramanantsoa said in a video statement released late on Tuesday that the former French national serviceman, Guy Baret, had been placed in pretrial detention at Tsiafahy maximum-security prison.

A Malagasy army officer, Colonel Patrick Rakotomamonjy, and other alleged accomplices were also implicated, he said.

According to Ramanantsoa, prosecutors have charged the suspects with spreading false information to disturb public order, plotting to sabotage infrastructure including power lines and thermal plants operated by state utility Jirama, harbouring wanted individuals and criminal conspiracy.

Authorities said the group had planned actions initially set for April 18, including power cuts, traffic disruption, efforts to incite security forces to rebel against state institutions and the mobilisation of youths to create unrest.

Prosecutors cited as evidence a WhatsApp group called "tolom-piavotan'ny olom-banona" ("Revolution of the Brave Citizens"), which they said was used to plan the operation.

The foreign ministry said French Ambassador Arnaud Guillois had been summoned and informed of the decision over the embassy agent. It did not identify the agent or specify the acts in question.

Reuters was not immediately able to contact the suspects or establish who their lawyers were.

Rakotomamonjy is awaiting presentation before an investigating judge. Two other suspects were placed under judicial supervision, with prosecutors saying they did not appear to be the masterminds of the conspiracy.