Pope John Paul II officially inaugurated the 15th World Youth Day celebrations at two huge open-air rallies in Rome yesterday evening.
About 600,000, including 2,000 Irish pilgrims, attended the gatherings at St John the Lateran and in St Peter's Square.
"Have no fear. Christ knows what is in the heart of man. Only He knows it," the Pope told a jubilant crowd at St John the Lateran. "Yes, let Christ reign over your young lives. Serving Christ is freedom."
He looked relaxed as he conversed with the enthusiastic crowd, interrupted from time to time by chants of "John Paul Two, we love you" and football-style anthems. "Thank you for your catechesis," he joked, telling them he saw "the hope of church and the world" shining in their eyes.
The rallies were the first event in a six-day programme of prayer, pilgrimage and meditation which will culminate on Sunday with a vigil followed by an open-air Mass in the countryside south of Rome. About 1.5 million young people are expected to attend.
"It's been totally amazing. I'm amazed at the number of people," said Mr Gerard Gallagher (31), one of the organisers of the national Irish pilgrimage, as he led a group into the vast throng outside St Peter's.
"There are 700,000 people expected to attend the Olympics, but here we will be more than a million. Looking around here there are flags from almost every country in the world. There is an element of nationalism, but what unifies people is what they believe."
Dana Rosemary Scallon was among the performers who entertained the crowd jammed into St Peter's Square as the Pope made his way by popemobile from St John the Lateran to the second international gathering.
By early afternoon Italian emergency services had answered more than 1,000 calls to assist young people who had fainted from heat and dehydration.
Sounding a contrary note, a Catholic writer and papal biographer, Mr Vittorio Messori, warned of the dangers of an excessive personality cult around the figure of the Pope and of "Catholicism with a guitar".