The Vatican said on Tuesday that it had placed under house arrest and opened criminal proceedings against one of its former ambassadors, Archbishop Jozef Wesolowski, who has been accused of sexually abusing boys he met on the street while serving in the Dominican Republic.

It is the first time the Vatican will hold a criminal trial on charges of child sexual abuse, and it comes as Pope Francis has been trying to set a new tone of rigorous attention in the long-running abuse scandal.

The case has received widespread attention in the Dominican Republic and in Mr. Wesolowski’s native Poland, and officials in both nations have sought to have him tried in their courts.

Mr. Wesolowski is being held “in a location within the Vatican City State,” according to a statement issued by the Rev. Federico Lombardi, the Vatican spokesman. He added that Mr. Wesolowski had presented documentation attesting to a medical condition, and was confined to house arrest after a preliminary hearing on Tuesday.