Denard, 76, staged a coup in 1978 while head of the powerful Comoran presidential guard, and ruled the Indian Ocean nation through figurehead presidents until the French forced him out in 1989.
Denard, whose real name is Gilbert Bourgeaud, suffers from Alzheimer's and other ailments, and was not required to attend - though he was on hand for the start of the proceedings.
The court rejected a move to require Denard to attend the trial, which is expected to last until 15 March. Denard left the courtroom after the court upheld a medical examination that paved the way for him to sit out the proceedings.
He briefly ruled the former French colony for a second time after toppling the government in September 1995, but French paratroopers arrested him a week later, citing a defence accord between France and Comoros. He was freed from a Paris prison in 1996.
The defendants on trial are accused of that coup attempt, and each faces up to ten years in prison if convicted of criminal association.
It is still unclear whether the French secret services were behind the coup or if Denard, who had extensive business interests in the Comoros, had acted to create a tax haven for money-laundering.
Denard, once considered France's top gun-for-hire, has led uprisings in Nigeria, Angola, Iran, Yemen and the Belgian Congo. He has said France often covertly supported his actions.
He was acquitted in 1999 of the assassination of Comoros president Ahmed Abdallah, who was shot dead in 1989.