A Catholic cardinal and five other people have gone on trial in France accused of covering up for a paedophile priest who abused boy scouts.

The case is France’s most important church sex abuse case to date amid growing demands in overwhelmingly Catholic France for a reckoning with decades of sexual abuse by the clergy.

Cardinal Philippe Barbarin, 68, appeared in a Lyon court on Monday along with other senior church officials accused of failing to protect children from alleged abuse by Bernard Preynat.

The top Vatican official in charge of sex abuse cases, Cardinal Luis Ladaria, is among the accused but will not appear in court because the Vatican invoked his diplomatic immunity.

After taking his seat in the front row, Barbarin held his hands together and closed his eyes, as if in prayer.

The judge then read out the lengthy accusations against the defendants, and the defence used procedural arguments to try to get the case thrown out. Nine people who said Preynat abused them in the 1970s and 80s brought the case, and hope it marks a turning point in efforts to hold the French church hierarchy accountable for hushing up abuse.

The victims say top clergy were aware of Preynat’s actions for years, but allowed him to be in contact with children until his 2015 retirement. Despite nationwide attention on the case, it may fall apart for legal reasons. Prosecutors initially threw it out for insufficient evidence, and Barbarin maintains his innocence.

His lawyer says his client never obstructed justice because the statute of limitations had passed on the acts in question by the time Barbarin was informed.

If found guilty of failing to report the priest’s actions, the defendants could face up to three years in prison and a 45,000 euro ($A72,184) fine. Barbarin and some other defendants are also charged with failing to assist a person in peril.

Preynat, now in his 70s, wrote letters to some families confessing the abuse, and is to be tried separately on sexual violence charges involving 10 children. One of his alleged victims, Alexandre Hezez, hailed the trial as an effort to “move justice forward”.

Hezez, 44, spoke to the cardinal directly about Preynat and is among those who brought the case to trial.

Barbarin sought counsel on how to handle abuse accusations against Preynat from Ladaria, who recommended disciplinary measures while “avoiding a public scandal”.

Numerous child sex abuse claims have been made against Catholic clergy in France since the 90s, but there has not been a huge wave like those seen in the US, Ireland or some other countries.

Barbarin is the highest-level French church figure accused of covering up abuse. Multiple cardinals have been accused in recent years of shielding abusers or committing abuse themselves, from Pennsylvania to Australia to Chile.

It comes as Pope Francis vowed justice for victims of clerical sex abuse on Monday, describing paedophilia as one of the “vilest” crimes ahead of a historic global meet on the crisis roiling the church.

“I cannot refrain from speaking of one of the plagues of our time, which sadly has also involved some members of the clergy,” he said in his annual address to ambassadors to the Holy See.

“The abuse of minors is one of the vilest and most heinous crimes conceivable. Such abuse inexorably sweeps away the best of what human life holds out for innocent children, and causes irreparable and lifelong damage,” he said.

Francis swore to “render justice to minors”, and said a meeting of the world’s bishops in February was “meant to be a further step in the church’s efforts to shed full light on the facts and to alleviate the wounds caused by such crimes”.

A litany of child sexual abuse scandals has rocked the Catholic Church, which has 1.3 billion followers around the world.