The most senior French Catholic cleric to stand trial over one of the paedophilia scandals that have rocked the church has denied in court covering up for a priest who abused boy scouts in the 1980s and 90s.

Cardinal Philippe Barbarin, archbishop of Lyon in southeastern France, is accused along with five others from his diocese of failing to report a priest under his responsibility to the authorities.

The 68-year-old bishop, a conservative who has made headlines with his blunt comments in the past, denied the allegations, which are the most damaging yet against the church in France.

"I never sought to hide, much less cover up these horrible acts," he told the court in Lyon, reading from a written statement.

He faces up to three years in prison and a fine of €45,000 if convicted of failing to report priest Bernard Preynat or remove him from positions involving care of children.

His former chief of staff Pierre Durieux, who is also on trial, complained in court that the defendants were the subject of a "witch-hunt".

Mr Durieux, a former scout, said he had been "particularly moved" by the story of a fellow scout who wrote to the cardinal in 2014 about Fr Preynat.

"Your story could have been mine," he told Alexandre Hezez, one of the nine victims who brought the case to court.

Cardinal Barbarin has been accused of being deaf to the suffering of abuse victims until they went public from 2015.

He has admitted that he had "not always used the best and most tactful words in the past".

After initially saying he first knew about the abuse claims in 2014, he admitted in 2016 to having known about them for nearly a decade.

He caused shock that year by voicing relief that most of the incidents over which Fr Preynat was accused were covered by the statute of limitations, meaning they could no longer be prosecuted - remarks he called a "language error".

The scandal in Lyon first blew up in 2015 when former scout Francois Devaux went public with allegations that Fr Preynat abused him as a child 25 years earlier.

Speaking shortly before the trial began, Mr Devaux said: "The goal is to stop the same mistakes being repeated."

Mr Devaux filed a police complaint against Cardinal Barbarin, who has retained the support of Pope Francis.

After six months of investigation and ten hours of interviews with the cardinal, investigators dropped the case in 2016, saying the allegations against him were either too old or impossible to prove.

But nine victims succeeded in reopening the probe, which led to him and the others, including the now-archbishop of Auch and the bishop of Nevers in France, having to stand trial.

All of them worked in the Lyon diocese at the time of the abuse.

Pope vows justice for victims of 'vile' sex abuse

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

"The abuse of minors is one of the vilest and most heinous crimes conceivable," he said in his annual address to ambassadors to the Holy See, swearing to "render justice to minors".

"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.

"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.

Pope 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.

In December, the pontiff had vowed the church would never again treat abuse allegations without "seriousness and promptness", calling on abusers to hand themselves in to police.

The Argentine pontiff has struggled to resolve the problem as the steady drip of scandal corrodes the church's authority amid sharp divisions in Rome over how to handle the fallout, and an apparently endless stream of bad news.

On Friday, the Vatican confirmed that an Argentinian bishop who has held a senior position at the Holy See since 2017, is under preliminary investigation over sex abuse claims.

Separately, during the same address to ambassadors, the Pope warned against a resurgence of nationalist and populist movements and criticised countries that try to solve the migration crisis with unilateral or isolationist actions.

Speaking to diplomats in an annual speech known informally as his "state of the world" address, the Pope suggested such movements and closed-door policies were turning the clock back 100 years to the dangerous period between the world wars.

Relationships within the international community "are experiencing a period of difficulty, with the resurgence of nationalistic tendencies," he said, making dialogue between countries and hurting the most vulnerable members of society, including migrants.

In the hour-long speech, Pope Francis several times mentioned the League of Nations, which was set up after World War I to promote peace but failed to stop the nationalist and populist movements that helped lead to World War II.

"The reappearance of these impulses today is progressively weakening the multilateral system," he told envoys from 183 countries in the speech, which touched on the situation in many countries.

While migration has led to diffidence and unilateral actions by governments, particularly in Europe and North America, the international community has a duty to defend refugees and migrants, he said.

"I do not believe that partial solutions can exist for so universal an issue," he said.