ST. LOUIS (AP) _ Welcomed like a rock star by a crowd of 20,000, Pope John Paul II warned America's young people against drifting into a world ``filled with darkness'' and rife with violence, drugs and easy sex.

``You are each called to be a disciple of Christ,'' he told a jubilant youth rally Tuesday night at a packed sports arena, where his somber message sharply contrasted with his listeners' shrieks of excitement.

The pope's 30-hour visit to the American heartland kept up a mission begun in Mexico to rally his flock against a ``culture of death'' as the church heads into the new millennium.

More than 100,000 people were expected to turn out today for his major public event in St. Louis, a Mass at the Trans World Dome. He is to return to Rome tonight.

During the youth rally at the Kiel Center, the crowd waved white handkerchiefs in welcome, chanting, ``John Paul II, we love you.''

``He's just such an incredible role model, someone you can really look up to,'' said Sarah Einhorn, 14, of Effingham, Ill.

Sports were a metaphor for his message, and the youths presented him with a hockey jersey that read: ``John Paul II No. 1.''

``I am told there was much excitement in St. Louis during the recent baseball season, when two great players, Mark McGwire and Sammy Sosa, were competing to break the home run record,'' he told the young people. ``You can feel the same great enthusiasm as you train for a different goal _ the goal of following Christ, the goal of bringing his message to the world.''

He turned somber when describing ``the darkness of children who go hungry and even die. The darkness of homeless people of who lack work and proper medical care. The darkness of violence.''

``There is something terribly wrong when so many people are overcome by hopelessness to the point of taking their own lives,'' the pope said. Instead, he said, ``Do not listen to those who encourage you to lie, to shirk responsibility. Do not listen to those who tell you that chastity is passe.''

The 78-year-old pope appeared tired during the evening event. His shoulders were slumped and his left hand trembled.

The trip was John Paul's fifth to the U.S. mainland, and his message contained a familiar theme _ that Americans should use their freedoms responsibly and strive for a higher moral standard.

During a ceremony at the airport upon his arrival Tuesday afternoon, he likened the U.S. debate over abortion and euthanasia to the one over slavery, calling the outcome a test of national character.

``America faces a similar time of trial today,'' he said with President Clinton seated nearby.

``Today, the conflict is between a culture that affirms, cherishes and celebrates the gift of life, and a culture that seeks to declare entire groups of human beings _ the unborn, the terminally ill, the handicapped and others considered `unuseful' to be outside the boundaries of legal protection.''

The pope is also a leading opponent of capital punishment and has intervened in several cases in the United States. Missouri's Supreme Court, without explanation, postponed an execution that was to have taken place while the pope was in town. Papal spokesman Joaquin Navarro-Valls called the postponement ``a mockery.''

During previous visits to the United States, home to 62 million Roman Catholics, the pope has spoken out against American materialism and consumerism.

At the airport, John Paul avoided certain sources of tension with the United States and dropped from the advance text of his speech any mention of land mines, drug trafficking or U.S. air strikes against Iraq, something the Vatican has assailed as ``aggression.''

``For 20 years you have challenged us to think of life not in terms of what we acquire for ourselves but what we give of ourselves,'' the president told John Paul at the airport ceremony. ``We honor you for standing for human dignity and human rights.''

Later, during a private 20-minute meeting between Clinton and the pope, abortion only ``came up in passing'' and the Monica Lewinsky affair not at all, White House spokesman P.J. Crowley said. The two did discuss Cuba and Iraq, Navarro-Valls said.