Former French mercenary Bob Denard has appeared in court accused - along with 26 others - of staging a coup attempt in the Comoros Islands in 1995.
Mr Denard has fought in several civil wars across Africa
The 76-year-old, who is suffering from Alzheimer's, is to be tried in absentia after the opening of the trial in Paris because of his illness.
The defendants are accused of criminal conspiracy during the coup in the tiny Indian Ocean nation.
The coup was ended within several days by French troops.
Mr Denard's lawyer said he had asked his client to be present at the opening of the trial "so that all can see his condition", according to the AFP news agency.
A court medical report found earlier in February that Mr Denard should be tried in absentia.
Prosecutors had been arguing that Mr Denard's presence in court would be essential.
During the coup in late September 1995, the defendants took hostage the then-President of the Comoros Islands, Said Mohamed Djohar.
The president was freed after France - acting under a co-operation treaty with the Comoros Islands - sent troops who arrested the mercenaries.
Mr Denard was arrested in 1995 but freed a year later
Mr Denard - once seen a France's top "dog of war" - has fought in civil wars in what is now the Democratic Republic of Congo, Yemen, Nigeria and Angola.
In 1989, he took part in a coup during which Comoros President Ahmed Abdullah was assassinated.
Mr Denard was acquitted of the murder at a trial in France in 1999.
The Comoros have experienced more than 20 coups or attempted coups, beginning just weeks after independence from France in 1975.