VATICAN CITY, Aug. 16--Pope John Paul II has welcomed more than 600,000 young Roman Catholic pilgrims from throughout the world to Rome with an appeal for "a time of peace in justice and freedom" for all people.

The 80-year-old pontiff spoke Tuesday at gatherings in the squares outsidethe Basilicas of St. John in Lateran and St. Peter to officially openthe church's Jubilee Holy Year celebration of the 15th World Youth Day.

Some 1.2 million youths are expected to gather during six days ofspiritual exercises and attend a papal Mass culminating the celebrationson Sunday.

The Vatican said that with young people continuing to arrive, the600,000 in Rome for the pope's welcome were 30 percent more thanexpected.

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 outonto surrounding streets Tuesday, the pope solemnly recited the names of the 159countries the pilgrims came from--45 in Africa, 29 in the Americas, 38in Asia, 43 in Europe and four in Oceania. Drums played in thebackground.

"With special affection, I greet the group of young people fromcountries where hatred, violence and war bring suffering to the life ofentire populations," he said. He thanked the young people fromindustrialized countries whose donations helped to pay for the trip toRome for youths from poorer countries.

"To them I say, in your name as well, that in our gathering we areclose to them as brothers and sisters; with all of you, I ask for themand for their people a time of peace in justice and freedom," the popesaid.

Noting that members of other Christian churches and ecclesialcommunities were present in the square with their pastors, the pope saidhe hoped World Youth Day also would be another step on the way toChristian unity.

The pope traced his own early life in appealing to the young peopleto let their faith "grow and be strengthened."

"I remember how as a child, in my own family, I learned to pray andtrust in God," he said. "I remember the life of the parish that Iattended, called after St. Stanislaus Krakow. It was runby the Salesian Fathers, from whom I received my basic training inChristian living.

"I cannot forget the experience of the war and the years of work ina factory. My priestly vocation came to its full maturity during theSecond World War during the occupation of Poland.

"The tragedy of the war gave a particular coloring to the gradualmaturing of my vocation in life," he said. "In these circumstances, Iperceived a light shining ever more brightly within me: the Lord wantedme 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 astrong voice with evident emotion, told the young people in Piazza St.John in Lateran earlier that he wanted to open the youth jubilee withthe 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 saidand then added. "Open your hearts, your lives, your doubts, yourdifficulties, your joys and your loves to his saving force and let himenter 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'vealready lived 80 years, and the young want me to be always young. Howcan I do it?"

From the Piazza of St. John in Lateran, he went to St. Peter'sSquare where hundreds of thousands of young people had waited more thantwo hours in hot and humid weather for his arrival.

The crowd showered the pope with flower petals, waved flags, bannersand scarves, sang, danced and repeatedly interrupted his brief greetingswith cheers and chants.

The pope drove to and from the Piazza of St. John in Lateran, thelargest in Rome and the traditional site for May Day rallies and rockconcerts sponsored by labor unions.