"This has been really cool," said Taylor Law, 19, who is about to enter university in his home town of Galveston, Texas, and who spent the night on the field with some 50 friends.
"Some of the countries the kids are from are in conflict but they are still here, showing that they are Catholic, that they believe in God as much each other does and this can only help peace," he said, waving an American flag as the Mass ended.
But this World Youth Day has been somewhat overshadowed by the issue of sexual abuse of minors by clergy.
Pope Benedict on Saturday apologised directly for the first time for sexual abuse of minors by Catholic clergy, but victims groups in Australia said they wanted action and not words.
The Pope, making some of his most explicit comments on the sexual abuse scandal that has rocked the Church in several countries, also said unequivocally that those responsible should be brought to justice.
In his homily at Sunday's concluding Mass, the Pope made an oblique reference to the scandal, saying the Church needed "renewal", but in his comments welcoming the Pope, Sydney's Cardinal George Pell was more direct.
"Too often she (the Church) is weighed down and burdened with the sins and failings of her children; too often she appears disfigured and discouraged," Pell told the Pope.
Broken Rites, which represents abuse victims in Australia, has a list of 107 convictions for church abuse, but says there could be thousands of victims as only a few cases go to court.
In Australia, home to the world's biggest gay and lesbian mardi gras, the Church's teachings often fall on deaf ears.
More than 5 million Australians describe themselves as Catholic, but less than 1 million attend Sunday Mass and the number may have dropped to about 100,000 in the past 5 years.
Around 1,000 protesters marched in Sydney on Saturday against Church teachings on sexual morality, trying to hand out condoms to pilgrims who were marching to see the Pope.
The Pope, who appears to be holding up well despite the long trip, leaves for Rome on Monday.
