The young Catholics had gathered for an evening vigil, an outdoor slumber party and morning mass.
Leo arrived by helicopter as the sun set over the Tor Vergata field and immediately boarded his open-topped popemobile for long loops through the flag-waving, cheering pilgrims.
They had already been partying there for hours, setting up campsites for the night as misting trucks and water cannons spritzed them to cool them down from the 30 degree temperature.
Leo displayed his fluency in speaking to the kids in Spanish, Italian and English about the dangers of social media, the value of true friendship and the need to have courage to make radical choices like marriage or religious vows.
"Friendship can really change the world. Friendship is a path to peace," he said.
"How much the world needs missionaries of the Gospel who are witnesses of justice and peace!"
But history's first American pope also alerted them to some tragic news.
Two young people who had made the pilgrimage to Rome had died, one reportedly of cardiac arrest, while a third was hospitalised, Leo told the crowd during the vigil service.
For the past week, these bands of young Catholics from around the world have poured into Rome for their special jubilee celebration, in a holy year in which 32 million people are expected to descend on the Vatican to participate in a centuries-old pilgrimage to the seat of Catholicism.
The young people have been traipsing down cobblestoned streets in colour-coordinated T-shirts, praying the rosary and singing hymns with guitars, bongo drums and tambourines shimmying alongside.
Using their flags as tarps to shield them from the sun, they have taken over entire piazzas for Christian rock concerts and inspirational talks, and stood for hours at the Circus Maximus to confess their sins to 1000 priests offering the sacrament in a dozen different languages.
It all has the vibe of a World Youth Day, the Catholic Woodstock festival that St John Paul II inaugurated and made famous in Rome in 2000 at the very same Tor Vergata field.
Then, before an estimated two million people, John Paul told the young pilgrims they were the "sentinels of the morning" at the dawn of the third millennium.
Officials had initially expected 500,000 youngsters this weekend, but Leo hinted the number might reach one million.
Those Romans who didn't flee the onslaught have been inconvenienced by the additional strain on the city's notoriously insufficient public transport system.
Residents are sharing social media posts of outbursts by Romans at kids flooding subway platforms and crowding bus stops that have delayed and complicated their commutes to work.
But other Romans have welcomed the enthusiasm the youngsters have brought.
Premier Giorgia Meloni offered a video welcome, marvelling at the "extraordinary festival of faith, joy and hope" that the young people had created.
"I think it's marvellous," said Rome hairdresser Rina Verdone, who lives near the Tor Vergata field and woke up on Saturday to find a gaggle of police outside her home as part of the massive, 4000-strong operation mounted to keep the peace.
"You think the faith, the religion is in difficulty, but this is proof that it's not so."
Verdone had already made plans to take an alternate route home on Saturday afternoon, that would require an extra kilometre walk, because she feared the "invasion" of kids in her neighbourhood would disrupt her usual bus route.
But she said she was more than happy to make the sacrifice.
"You think of invasion as something negative. But this is a positive invasion," she said.
