Members of a U.N. committee on torture questioned Vatican officials for two hours about the church's handling of abuse cases, with one member claiming a "climate of impunity" existed within the Vatican.
The hearing marked the Vatican's first appearance before the committee after it signed in 2002 an international convention banning torture and cruel, inhumane and degrading treatment.
One legal expert said the hearing might spark further prosecutions of priests.
"Torture comes with universal jurisdiction," she said. "States which have signed up to the U.N. convention are obliged to follow up, and there is no statute of limitations and acquiescence in torture is very serious."
The children's rights committee criticized the Vatican for "systematically" protecting predator priests, allowing "tens of thousands" of children to be abused. It also questioned core Catholic teachings on homosexuality, contraception and abortion.
That prompted a Vatican spokesman on the eve of Monday's hearing to argue that the U.N. risked "losing authority" as it succumbed to pressure exercised by nongovernmental bodies with a "strong ideological character."
But he has set up a new commission on sexual abuse, including Cardinal Sean O'Malley of Boston, who said Saturday that senior church members were still "in denial" over the phenomenon.
Joining O'Malley on the commission is Marie Collins, an Irish woman who was abused by a priest when she was 13.
Collins has campaigned against abusive priests but said Saturday that since the U.N. committee dealt with state-sponsored torture she felt it was not the place to discuss the issue.
"I don't believe it is connected in any way to the work we are doing," she said.