Mr. Napolitano was said to have defended the morals and professional stature of Mr. D’Ambrosio. He also said he had little information to add to the trial itself.

In a note, the president’s office said that Mr. Napolitano had “answered all questions” and that it was the expressed hope of the president that the transcripts of the testimony, “given by the head of state with the utmost transparency and serenity,” would be made public as soon as possible.

Prosecutors are investigating whether the state came to an accord with mob bosses to halt a spate of violence, including bombings at the famed Uffizi museum in Florence, a museum in Milan and churches in Rome.

Prosecutors claim that talks between the mobsters and the state began two months before the anti-Mafia prosecutor Giovanni Falcone, his wife and three bodyguards were killed in May 1992 in an explosion on a Sicilian highway just as they were crossing it. Two months later, a car bomb killed another anti-Mafia magistrate, Paolo Borsellino, and his bodyguards in Palermo.

Prosecutors believe the easing of conditions for about 300 jailed mobsters at that time is evidence of an accord between the state and the Mafia.

The decision to ask Mr. Napolitano to be a witness in the trial was controversial because of his standing. It gave rise to public debate, as well as feverish news media coverage. Mr. Napolitano’s testimony “is or can be a dramatic moment in our institutional history, above all because an opaque and dangerous operation seems to be underway to let the public opinion believe that Napolitano” is somehow on trial, Stefano Folli, an editorialist for the newspaper Il Sole 24 Ore, said in a radio commentary.

Mr. Folli cautioned the Palermo prosecutors not to transform the trial into a “media forum that exploits the Quirinale to set up a great circus destined in the end to evaporate like a soap bubble, leaving behind only the rubble of institutional credibility.”