But the date chosen was hardly auspicious: the anniversary of the St Bartholomew's Day massacre, when thousands of French Protestant Huguenots were murdered by Catholic militia.
The Pope sought on Saturday to allay the controversy over the killings, on 24 August 1572, by referring to the Catholic role and appealing for inter-religious unity. "On the eve of August 24, we cannot forget the sad massacre of St Bartholomew's Day, an event of very obscure causes in the political and religious history of France.
"Christians did things which the Gospel condemns," he said in a vigil before the Mass, during which he pleaded with young people to revive faded Roman Catholic faith in the West. "Belonging to different religious traditions must not constitute today a source of opposition and tension," he said. He made no further mention yesterday of the massacres, which came during civil wars.
The 77-year-old Pope, who has looked weary in the sweltering heat, smiled and looked at ease as the young pilgrims repeatedly cheered him. However, he hinted at his tiredness and age, saying: "The longer we live, the more we realise how precarious life is, and the more we wonder about immortality."
Many of the young people had spent the night camping at the racecourse or in the nearby Bois de Boulogne at the end of the World Youth Days festival.
"Dear young people, your journey does not end here ... go forth now along the roads of the world, along the pathways of humanity, while remaining ever united in Christ's Church," said the Pope, who invited them to the next World Youth Days in Rome in 2000, which he has declared a holy year at the start of the millennium.
Church officials feared before the festival that the turn-out could be embarrassingly low. But the crowd was one of the biggest the Pope has drawn recently and on a par with that during a visit to his Polish homeland last June.
Opinion polls coinciding with the trip, however, show that French people increasingly consider the church irrelevant and the Pope's conservative morality out of step with modern life. But the vast turn-out was a boost for the church after the Pope had clashed with France's ruling Socialist Party over his visit to the grave of his friend, Jerome Lejeune, a leading geneticist and anti-abortion campaigner.
The Pope also raised a French female saint to a privileged place in the male-dominated church. He said that he would make Saint Therese of Lisieux a doctor of the church at a ceremony at the Vatican in October.
Doctors of the church are people of great holiness whose teaching or spirituality has had a profound impact on the life of the church.
Saint Therese will be the 33rd such doctor, but only the third woman, joining Saint Teresa of Avila and Catherine of Sienna, who were raised to the position in 1970. No one has been made a doctor of the church since then and the Pope has never before conducted the ceremony.
Saint Therese, who died of tuberculosis 100 years ago aged 24, is a co- patroness of France alongside the Virgin Mary, Joan of Arc and Saint Martin. She was made a saint in 1925.