Greeting the new year with an appeal for peace and a party for tens of thousands of children, Pope John Paul II has embarked on a crowded Jubilee schedule that will test his endurance to the limit.

The Christmas and New Year's festivities were only the start of a year-long calendar of Holy Year events for the 79-year-old Roman Catholic pontiff, who wages a constant battle against the debilitating effects of a neurological ailment.

John Paul plans at least one pilgrimage to the Holy Land this year and scores of special meetings with Holy Year pilgrims expected to flock to Rome and the Vatican by the hundreds of thousands.

The Vatican's Central Committee for the Great Jubilee of the Year 2000 has scheduled mass pilgrimages by the sick, the elderly and the disabled, as well as artists, artisans, scientists, immigrants, journalists, families, athletes, politicians, farmers, soldiers and university professors.

The pope also will preside over beatifications and canonizations in St. Peter's Square, and he will visit Rome's Rebibbia Prison for a Holy Year celebration with its inmates.

The Vatican welcomed its first mass of Holy Year pilgrims Sunday when a crowd of 120,000 adults and children gathered in St. Peter's Square and on the wide avenue leading from the Tiber River to the square for a children's jubilee.

"There are so many of us, you can't see where it ends, perhaps to the Tiber and beyond," a smiling pope commented.

"You belong to the third millennium, not the last one," John Paul told some 50,000 children, ages 6 to 14, who had traveled to the Vatican from 30 countries. Among them were 10 former child soldiers rescued by the church in Sierra Leone.

Speaking in Italian, French, English, Spanish and his native Polish, John Paul deplored poverty, violence and the exploitation of children and attacked abortion and artificial birth control.

"At the beginning of a new year, dear children and young people, we cannot forget all those of your own age who are suffering hunger or violence and those who are victims of hideous forms of exploitation," he said. "How can we forget the many children who are denied even the right to be born?"

Two days earlier, at a New Year's Eve Te Deum Mass of Thanksgiving in St. Peter's Basilica, John Paul singled out the discovery of America and the rise of the "oppressive ideologies" of fascism and communism as major events of the 20th century.

On New Year's Day, the pope drove across the Tiber to the 13th-century Basilica of St. Mary Major, atop the Esquiline Hill, to mark the church's 32nd annual World Day of Peace. He formally opened the basilica's Holy Door and celebrated Mass.

John Paul returned to the theme of peace during the Sunday Angelus prayer from his study window. "An invocation of peace rises in chorus from every part of the world," he said. "We pray that it does not fall unheard. In this moment, my thought goes to the many victims of violence, to those who feel alone and abandoned."

On Thursday, John Paul marked Epiphany--the 12th day of Christmas and the celebrated arrival date of the Magi in Bethlehem--by consecrating newly elevated bishops and archbishops. Tomorrow, the Feast of Jesus's Baptism, he will baptize a group of infants.

The pope's holiday observances will close Monday when he delivers an address on international issues at his New Year audience for diplomats from more than 150 countries accredited to the Vatican.