The Pope presided over the Mass on Thursday, held outside the Basilica of St. John Lateran, and he joined in the eucharistic procession that followed.
Unlike in earlier years, the Holy Father delegated the celebration of the open-air Mass to Cardinal Camillo Ruini, his vicar for Rome.
But the Pope delivered the homily, a meditation on the eucharistic mystery. "The days, years and centuries go by, but not this most holy gesture, in which Jesus has condensed all his Gospel of love," he said.
"On this feast, every city -- the metropolis as well as the smallest village of the world -- spiritually becomes Zion, the Jerusalem that praises the Lord," the Pope continued.
"This people needs the Eucharist. The Eucharist makes the Church be missionary, but can this take place without priests who renew the eucharistic mystery?" the Pontiff asked.
At the end, John Paul II addressed the youth who were present. "If any one feels the Lord´s call to give himself totally to him, to love him with an undivided heart, let him not be held back by doubt or fear," he urged.
"May he express his own ´yes´ without reservations, trusting him who is faithful to all his promises!" the Holy Father added.
John Paul II followed the Mass from his throne, placed at the right side of the altar, before tens of thousands of faithful.
During the liturgy, those present prayed "for the peoples of the Holy Land and for all those who live the drama of war, oppression and social injustice, so that the Lord will uproot fear, abate disputes, inspire farsightedness in rulers and the will to seek just solutions that respect the dignity of every man."
The Mass was followed by the procession from the Basilica of St. John Lateran to St. Mary Major, linked by Via Merulana, which extends for about a kilometer.
Windows of buildings along the way were decorated with candles and tapestries. John Paul II knelt on a prie-dieu during the procession, in adoration of the Eucharist, exposed in a monstrance, on the platform of a soberly decorated white truck.
John Paul II reintroduced the Corpus Christi procession through the streets of Rome the first year of his pontificate. The custom was abandoned following the annexation of the Papal States in 1870, which for years confined popes within the Vatican walls.