Pope John Paul II forcefully reaffirmed the Catholic Church's unflagging opposition to divorce Saturday, saying a stable marriage was the foundation of all social order.
Earlier in the day, the Vatican also moved quickly to quash a German newspaper report that surgery was being considered for the pope's right leg, which he broke in a fall. The 77-year-old pontiff has limped since a hip-replacement operation.
"At the base of every social order you will find this principle of unity and indissolubility of marriage -- the principle upon which the institution of the family and all family life is based," the pope said during the homily of a Mass in the modernistic Rio de Janeiro Cathedral.
The pope in the past has expressed concern over new legislation in several Latin American countries that would facilitate marital break-ups.
"Marriage, to be parents, motherhood, fatherhood, all this belongs to the order of nature, from the time when God created men and women," the pope said.
The previous evening he spoke to delegates at a conference on family values of the "necessary diversity and sexual complementarity" of a married couple.
This appeared to be an attack on homosexual marriages, which the Catholic Church firmly opposes as a threat to the traditional family.
The Brazilian Congress is currently debating a controversial bill that would give homosexual partners the same inheritance rights as traditional married couples.
The pope is in Brazil to preside over an international meeting on families that has attracted thousands of pilgrims from around the world.
Earlier Saturday, the Vatican firmly denied news reports that it was talking to a German surgeon about carrying out an operation on the pope.
"The cited doctor . . . has never visited the Holy Father," chief Vatican spokesman Joaquin Navarro-Valls said. "The hypothesis that is being referred to -- surgical operation to correct a functional insufficiency of the right hip -- has never been taken into consideration."
The pope, who has had a series of health problems in recent years, has appeared in fine form during his Brazilian trip.
"He is healthy and in good spirits," Navarro-Valls told reporters, adding that the pope had joked with aides Saturday morning about how he would have liked to travel outside Rio de Janeiro.
Organizers have deliberately set up a light schedule so as not to exhaust the pontiff.
Among the specially invited guests to the service was Maria do Carmo Geronimo, who was born a slave 126 years ago and is considered one of the world's oldest people.
Security at the service was tight, with guards checking that the photo appearing on every participant's identification card matched the computer image called up by scanning a bar code.
The high point of the pope's visit will be an open-air mass today on Rio de Janeiro's beachfront esplanade, which organizers expect to be attended by more than 1.5 million people.