The unidentified man doused himself with gasoline he had carried in a bottle and then lit himself aflame at the end of the colonnade near Piazza Pio XII at 8:30 in the morning Dec. 19.
A Jesuit priest on his way to work at the Vatican was the first passer-by to come to the man's aid, according to a written statement from Italian police that was then distributed by the Vatican press hall.
The priest tried to smother the flames with his cloak and jacket. Two Italian police agents on duty at the square went to the scene and used a blanket and fire extinguisher to put out the fire.
The man was hospitalized with severe burns on his upper body and the two agents were treated and released for "respiratory problems and burns to the hands," the statement said.
The statement said a piece of paper was found nearby that had the phone number of the man's daughter. It added that the motive for the man's actions was unknown.
The case was in the hands of the Italian authorities because security and law enforcement in St. Peter's Square are the responsibility of the Italian police.
In 1998, a 40-year-old Italian man set himself on fire with gasoline in the square on an early morning in mid-January. The man died less than two weeks later after suffering from third-degree burns over 90 percent of his body.
There was another moment of tension Dec. 19, a few hours after the burning incident, when an unidentified woman was escorted away by police from outside the Vatican press hall, several yards from the square. The blond-haired woman was screaming in what some bystanders said seemed to be a Slavic language.