Leo, addressing crowds stretching out over a large field near the famed Catholic shrine in Lourdes, said the church must have the courage to face its past but did not directly reference abuse scandals in France that sparked nationwide shock.
"In order to heal the deep wounds, infidelities and painful errors that have occurred within (the church), we must humbly place ourselves under the Lord's purifying gaze," the first US Pope told an estimated 150,000 people gathered in the field on a sunlit day.
Leo is on a whirlwind three-city, four-day visit to France, where he has again urged the world to reconsider the development of artificial intelligence.
He drew about 800,000 people for an outdoor mass on the Champs-Elysees in Paris on Saturday.
Local church leaders are hoping he will help repair the church's standing after a 2021 report found some 330,000 French children had been abused by Catholic clergy and lay leaders over seven decades.
In an earlier, closed-door meeting with French bishops on Sunday, Leo did directly refer to "the painful scourge of abuse committed against minors by members of the clergy", according to a Vatican-provided text.
The Pope thanked the bishops for the "courageous measures" he said they had taken to confront the scandals and urged them to buttress their safeguarding efforts and "continue on the path undertaken, maintaining the greatest vigilance".
Leo has previously apologised directly for abuse scandals that have damaged the church's credibility across the world.
On a trip to Spain in June, he told bishops there they must listen to survivors and offer them reparations.
The Pope is due to have a closed-door meeting with seven French victims of abuse by clergy later on Sunday.
Some survivors said they were doubtful the encounter would result in quick changes by the church.
"I am quite sceptical that the Pope will solve the issue, at least not immediately," Michele Le Reun-Gaigne, co-president of an association for survivors in western France, told Reuters.
"Things are moving very slowly," she said.
"It isn't going to happen overnight."
Bishop Pierre-Antoine Bozo, the spokesperson for the French bishops, said the Pope's desire to have the meeting "shows that we cannot ignore this issue and that it must always remain among our concerns".
The Pope's itinerary in France includes six events in Lourdes, a town in the foothills of the Pyrenees mountains that attracts some three million pilgrims a year to its shrine to the Virgin Mary and purported healing spring waters.
The site is also connected to a scandal that has dogged the Vatican in recent years.
The shrine's facade has a series of mosaics by Reverend Marko Rupnik, a prominent religious artist with Vatican ties, who has been accused of sexual abuse.
Rupnik, accused by more than 20 women, is not known to have commented publicly on the accusations.
Five of his alleged victims wrote an open letter to the Pope on Thursday, sharply criticising an ongoing Vatican trial of the cleric.
The mosaics, storeys tall, have been covered for the Pope's visit with large, muted yellow tarps.
