About 300,000 Dominicans flocked to the outskirts of this tropical city today and heard Pope John Paul II chastise exponents of "liberation theology" who "see the poor as a class or as a class in struggle," and who forget that "the first liberation to reach for is the liberation from sin."

The pope spoke out on the controversial issue of liberation theology during an outdoor mass at the start of his 23-hour visit here. This activist doctrine, developed in Latin America, holds that poverty is a social sin that the church has an obligation to combat. Many of its exponents use Marxist methods of social research and analysis. It is this aspect of liberation theology that the Polish-born pope has singled out for criticism.

This is the pope's second visit to the Dominican Republic, a country of 6 million people that is, like most Latin American countries, overwhelmingly poor and overwhelmingly Catholic. He flew here from Spain, where he ended an overnight visit by meeting with Prime Minister Felipe Gonzalez.

The route from the airport to the race track where he said mass took John Paul past some of the city's worst slums, clusters of tin and cardboard shacks clinging precariously to muddy hillsides near the dank green Ozama River.

During the mass, John Paul acknowledged the urgent needs of the poor. "The pope and the church . . . want to be present in the cause of the poor," he said, and after commenting approvingly on a recent criticism of liberation theology issued by the Vatican's Sacred Congregation of the Doctrine of the Faith, he added that "those who work with laudable generosity in the cause of the poor should feel not braked but encouraged."

The pope's visit is to mark the beginning of holy years leading up to the 500th anniversary of Columbus' 1492 arrival and the evangelization of the Americas. More than 1,700 Latin American bishops gathered here today to greet the pope and plan the celebration.

The first day of John Paul's visit to the oldest city in the Western Hemisphere also included a visit to the continent's oldest cathedral and a brief meeting with Dominican President Salvador Jorge Blanco.