The trial of Pascal Simbikangwa - who denies all accusations against him - is being closely watched in France, which has long stood accused of failing to rein in the Rwandan regime at the time of the 100-day genocide in 1994.
The 54-year-old defendant appeared in court in a wheelchair after a 1986 car accident that left him a paraplegic. He faces life in prison.
Arrested in 2008 on the French Indian Ocean island of Mayotte, he is accused of inciting, organising and aiding massacres during the genocide, particularly by supplying arms and instructions to militia who were manning road blocks and killing Tutsi men, women and children.
After his arrest, France refused to extradite him to Rwanda, as it has done in previous cases, and decided to try him under laws that allow French courts to consider cases of genocide, crimes against humanity and war crimes committed in other countries.
The trial is expected to last six to eight weeks and, in a rare case for France, will be filmed, with recordings available once the case is concluded.
"It is history being made. We have always wondered why it has taken 20 years... It is late, but it is a good sign," he said.
Simbikangwa acknowledges being close to the regime of Hutu president Juvenal Habyarimana, whose assassination on April 6, 1994, unleashed the genocide, in which most of the victims were members of the minority Tutsi community.
But he denies participating in or organising massacres.
He was initially charged with genocide and crimes against humanity but the charges were downgraded to complicity.
His lawyers have attacked the prosecution's case as being based purely on unchallenged witness accounts.
But Simon Foreman, a lawyer who represents civil parties in the case, said the charge of complicity "in no way diminishes the responsibility" of Simbikangwa, whom he described as "a cog in a mechanism operated by others".