Madagascar has arrested 21 more suspects, including 12 military personnel, in connection with a plot to kill President Andry Rajoelina and topple the government, a senior prosecutor said.

Six people, one of them a French citizen, were arrested last month on suspicion of involvement in the plot, following what officials described as a months-long investigation in the Indian Ocean island.

The military personnel who were arrested include five generals from the army and the gendarmerie and two captains and five non-commissioned officers, Berthine Razafiarivony, of the Antananarivo court of appeal, said late on Sunday.

She did not give their identity.

Four retired foreigners and five civilians were also arrested, some of whom were being held in police custody, the prosecutor said.

"The physical evidence in the hands of the investigators is tangible and made it possible to identify the main instigators of the operation," Razafiarivony said.

Madagascar has a history of political violence. Rajoelina first seized power in the deeply impoverished former French colony of 26-million in a March 2009 coup, ousting Marc Ravalomanana.

He remained in control at the head of a transitional government until 2014. In the 2018 elections, Ravalomanana challenged Rajoelina, lost, and cried fraud.