ANTANANARIVO, July 23 (Reuters) - A French citizen is among six people arrested on suspicion of involvement in a plot to kill Madagascar's president, the Indian Ocean island's public security minister said, and a second official said the president's security had been tightened.

"One of the arrested people is French, two of them are bi-national - Malagasy and French. The three others are Malagasy," Rodellys Fanomezantsoa Randrianarison told a news conference late on Thursday.

Madagascar's attorney general said on Thursday police had arrested the six following what officials said was a months-long investigation.

Patrick Rajoelina, an adviser to President Andry Rajoelina, told Reuters on Friday that two of those arrested had previously worked in the French military.

The French Foreign Affairs Ministry said it had been informed of French nationals' arrests and that they could get consular help if they asked for it.

A spokesperson for the French military did not immediately answer a request for comment.

Patrick Rajoelina added that unspecified measures had been taken to tighten the president's security. "The evidence is tangible and we certainly do not take this lightly," he said.

Madagascar has a history of political violence and instability. Andry Rajoelina, 44, was sworn in as president in 2019 after a hard-fought election and a constitutional court challenge from his rival.

Rajoelina first took power in the deeply impoverished former French colony of 26 million people in a March 2009 coup, removing Marc Ravalomanana. He remained in control at the head of a transitional government until 2014.

In the 2019 elections, Ravalomanana challenged Rajoelina, lost, and said the vote was fraudulent.