The case is a historic one in a country which has been accused of failing to rein in the regime in power in Rwanda at the time of the slaughter and subsequently dragging its feet over the repatriation or prosecution of individuals suspected of involvement in crimes against humanity.
The trial of Pascal Simbikangwa, 54, opened on Tuesday in Paris and is expected to run for between six and eight weeks. Exceptionally in France, the court proceedings will be filmed.
Simbikangwa, who has been a paraplegic since a car accident in 1986, faces charges of complicity to commit genocide, based on incidents in Kigali and the northwestern Gisenyi region.
Simbikangwa, who denies all the charges against him, is accused of supplying arms, instructions and encouragement to Interahamwe Hutu militia who were manning road blocks and killing Tutsi men, women and children as they arrived.
But he will not be tried for the April 1994 massacre in Kesho, northwestern Rwanda, where he had a farm and is remembered as a man who hated Tutsis and inspired fear.
Simbikangwa acknowledges being close to the regime of the 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.
His lawyers, Alexandra Bourgeot and Fabrice Epstein, have attacked the prosecution case as being based purely on unchallenged witness accounts and stress that their client has always maintained that he does not understand why he is on trial.
He was initially charged with genocide and crimes against humanity but the charges against him were downgraded to complicity. A separate allegation of involvement in torture was dropped on statute of limitations grounds.
The former soldier's lawyers say his trial is the result of political-diplomatic pressure on the judiciary against the background of a rapprochement between France and the current government in Rwanda following three years (2006-09) during which diplomatic relations were broken off.
"There is pressure from the Rwanda authorities on France and monstrous pressure from the civil parties who initiated the case," the lawyers said in a statement.
"We have the impression that because it is the 20th anniversary of the genocide against the Tutsis, Pascal Simbikangwa has to be convicted as an example.
"We are going to do all we can to ensure he is not made a scapegoat, and we expect the court to judge Pascal Simbikangwa as a human being on the basis of precise facts."
In concrete terms, the accused is said to have organised roadblocks in the Rwandan capital Kigali and his native region of Gisenyi, the province Habyarimana also hailed from.
The checkpoints allegedly served to filter out Tutsis for execution and Simbikangwa is accused of providing arms and instructions to those who carried out the killings.
Prosecutors abandoned an attempt to implicate him in a massacre at Kesho Hill in Gisenyi province because of the retrospective and contradictory nature of witness statements directly related to him. Some 1,400 Tutsis were killed at Kesho, many of them in a church where women, children and the elderly had taken refuge.
Alain Gauthier, the chairman of the group of civil parties in the case (CPCR), described the decision not to include Kesho in the indictment as heartbreaking.
Simon Foreman, a lawyer who represents the CPCR, 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".
The CPCR's legal complaint resulted in Simbikangwa being arrested in October 2008 on the French Indian Ocean island of Mayotte, where he had been living under an assumed identity for three years.
France's courts refused, as they have done systematically, to extradite him to Rwanda for trial there.
Four other NGOs are also civil parties to the case but the only directly affected plaintiff, a Belgium-based Rwandan who holds Simbikangwa responsible for the massacre of her entire family, had her case rejected for lack of concrete evidence.
"There could have been more effort made to find direct victims," said Patrick Baudoin, a lawyer and the chairman of one of the NGOs involved, the International Federation of Human Rights (FIDH).
But he welcomed the fact a trial was finally taking place after years in which, "the evidence suggests there was not the political will" to bring cases to court.
"Finally France is meeting its obligations," he said.