The 80-year-old pontiff spoke Tuesday at gatherings in the squares outside the Basilicas of St. John in Lateran and St. Peter to officially open the church's Jubilee Holy Year celebration of the 15th World Youth Day.
Some 1.2 million youths are expected to gather during six days of spiritual exercises and attend a papal Mass culminating the celebrations on Sunday.
The Vatican said that with young people continuing to arrive, the 600,000 in Rome for the pope's welcome were 30 percent more than expected.
Wednesday, tens of thousands of the young pilgrims waited under a blistering sun in lines snaking from St. Peter's almost to the Tiber River for a chance to pass through the basilica's holy doors.
By mid-afternoon, when temperatures had soared to 95 degrees Fahrenheit, volunteers had handed out 200,000 liters of bottled water to the faithful and firefighters were opening hydrants to hose down the crowd.
Despite the heat, the pilgrims streamed into the avenue leading up to St. Peter's Square, singing with undimmed enthusiasm and waving huge flags.
``We were steaming,'' said Ron Biamonte, a 17-year-old pilgrim from Chicago. ``But I'd do it two more times just to go through.''
Passing through the holy doors is an essential part of the pilgrimage during the World Youth Day celebrations. Pilgrims are assigned a specific time for the ritual.
The holy doors are opened only when the Roman Catholic Church celebrates a Holy Year and are not main entrances into St. Peter's.
Addressing the crowd that filled St. Peter's Square and spilled out onto surrounding streets Tuesday, the pope solemnly recited the names of the 159 countries the pilgrims came from--45 in Africa, 29 in the Americas, 38 in Asia, 43 in Europe and four in Oceania. Drums played in the background.
"With special affection, I greet the group of young people from countries where hatred, violence and war bring suffering to the life of entire populations," he said. He thanked the young people from industrialized countries whose donations helped to pay for the trip to Rome for youths from poorer countries.
"To them I say, in your name as well, that in our gathering we are close to them as brothers and sisters; with all of you, I ask for them and for their people a time of peace in justice and freedom," the pope said.
Noting that members of other Christian churches and ecclesial communities were present in the square with their pastors, the pope said he hoped World Youth Day also would be another step on the way to Christian unity.
The pope traced his own early life in appealing to the young people to let their faith "grow and be strengthened."
"I remember how as a child, in my own family, I learned to pray and trust in God," he said. "I remember the life of the parish that I attended, called after St. Stanislaus Kostka...in Krakow. It was run by the Salesian Fathers, from whom I received my basic training in Christian living.
"I cannot forget the experience of the war and the years of work in a factory. My priestly vocation came to its full maturity during the Second World War during the occupation of Poland.
"The tragedy of the war gave a particular coloring to the gradual maturing of my vocation in life," he said. "In these circumstances, I perceived a light shining ever more brightly within me: the Lord wanted me to be a priest. I remember with feeling that moment in my life when, on the morning of 1 Nov. 1946, I was ordained a priest."
John Paul, who moved slowly, leaning on a cane, but spoke in a strong voice with evident emotion, told the young people in Piazza St. John in Lateran earlier that he wanted to open the youth jubilee with the same words he spoke On Oct. 22, 1978, at the start of his papacy.
"Do not be afraid. Open, open wide your doors to Christ," he said and then added. "Open your hearts, your lives, your doubts, your difficulties, your joys and your loves to his saving force and let him enter your hearts."
The crowd broke into prolonged applause and shouted, "Viva il papa (Long live the pope). Viva, il papa, viva."
John Paul gave the young people a quizzical look, and said: "I've already lived 80 years, and the young want me to be always young. How can I do it?"
The crowd showered the pope with flower petals, waved flags, banners and scarves, sang, danced and repeatedly interrupted his brief greetings with cheers and chants.
The pope drove to and from the Piazza of St. John in Lateran, the largest in Rome and the traditional site for May Day rallies and rock concerts sponsored by labor unions.
On papal trips, John Paul customarily rides through the cities he visits in his glass-sided popemobile, waving to crowds that line the streets, but this was the first time he has crossed the center of Rome in the popemobile rather than a limousine.
Aug. 15 is the Roman Catholic Feast of the Assumption and in keeping with tradition most shops and restaurants were closed and Romans were at the beach or in the mountains.
But in addition to the young pilgrims arriving for World Youth Day, thousands of Holy Year pilgrims and tourists lined the pope's route, which took him past the Colosseum, the Imperial Forums, the white marble Victor Emmanuel II monument to the Tiber River and the Vatican.
At the foot of Via della Conciliazione, the broad avenue leading to St. Peter's Square, the pope switched from the popemobile to the open white jeep he uses to circle the square during his weekly Wednesday general audiences.
The World Youth Day celebrations are the largest so far of the current Holy Year observances.
The Vatican and the Italian bishops conference mobilized an international force of 25,000 young volunteers to help with logistics. Thousands of boy and girl scouts were stationed in 780 white tentlike gazebos throughout Rome to sell religious literature, snacks and souvenirs.
A force of cardinals, archbishops and bishops will lead catechism sessions in 32 languages and 2,000 priests, working two-hour shifts, will hear confession and celebrate Mass in the Circus of Maximus where the ancient Romans competed in chariot races.
Those young people who could afford it paid 240,000 lire ($120) each for six days of housing with families or in Catholic institutions and gave an additional $10 contribution to help pilgrims from poorer countries.