The crowd roared when Pope Benedict XVI appeared on giant TV screens in a live hookup from St. Peter's Square in Vatican City, praising the crowd.
The pope, speaking during the traditional noon Sunday Angelus prayer, said the family is "based on the unbreakable union of man and woman and represents the privileged environment where human life is welcomed and protected from the beginning to its natural end."
"It is worthwhile to work for the family and marriage because it is worthwhile to work for the human being, the most precious being created by God," the pope said, speaking in Spanish. He urged parents to bring up their children with respect for the moral values that give dignity to human life.
It was Benedict's latest appeal for the traditional family, a central theme of his papacy. The Vatican has campaigned against proposals to legalize gay marriage, and denounced Prime Minister Jose Luis Rodriguez Zapatero's government for passing a law recognizing gay marriage.
The rally filled a central intersection, Plaza de Colon, and spilled over into neighboring streets. No crowd estimates were available.
The president of the Spanish Bishops Conference, Ricardo Blazquez, said the term "traditional family" often is interpreted as an anachronism.
But, he said, the traditional family "is rooted in human nature itself."
"Its validity is a thing of yesterday, today and tomorrow," he said.