French-Malagasy dual citizen Paul Rafanoharana, who local media reported was once an adviser to the president, was handed the highest punishment in the trial for conspiring to kill his former boss.

Rajoelina began a second presidential term on the Indian Ocean island in 2019.

Frenchman Philippe Francois, a former officer in the French military, was jailed for 10 years.

Another 14 people have been acquitted.

A high court judge in the capital Antananarivo said the six, who also included a retired Malagasy army general, were guilty of charges including plotting to overthrow the government and criminal association with a view to threatening the president's life.

All had denied the charges.

Maitre Willy Razafinjatovo, one of the lawyers for Rafanoharana, told Reuters he would appeal the ruling in a higher court.

"We are going to... have this trial set aside," he said.

"I am appalled. It's scandalous," another lawyer for Rafanoharana, Maitre Arlette Rafanomadio, said of the ruling.

The suspects were arrested in July and August of this year but details of the plan have not been made public.

Rajoelina, 47, first seized power in the former French colony of 26 million people in a March 2009 coup, ousting Marc Ravalomanana.

He remained in control at the head of a transitional government until 2014.

Australian Associated Press