The trial began Saturday for Pope Benedict XVI's former butler over the alleged leaking of hundreds of secret papers from the pope's personal apartment to an Italian journalist.
The butler, Paolo Gabriele, could face a sentence of up to eight years in an Italian prison if convicted, although it is possible the pope could choose to pardon him. He is charged with aggravated theft.
Gabriele did not enter a plea but has admitted leaking the papers to the Vatican prosecutor, according to Vatican statements.
Vatican computer technician Claudio Sciarpelletti, who worked in the Vatican's secretariat of state, is also on trial, accused of complicity in the crime. If found guilty, he faces a shorter prison term of only a few months.
The Gabriele case is thought to be the most significant ever heard in the Vatican City courthouse, which has handled mostly petty theft cases in the past.
Corruption claims based on the leaked documents rocked the Catholic Church hierarchy and could even affect who becomes the next pope.
Saturday's initial session was held under closely controlled conditions, with only a handful of approved reporters allowed to attend.
Those reporters later briefed other journalists on the proceedings. They had been made to hand over their own pens in exchange for Vatican-issue ones in case any contained concealed listening devices, the reporters said.
They recounted that the three lay judges, led by Giuseppe Dalla Torre, heard that Vatican investigators had seized 82 boxes of various sizes of evidence from Gabriele's apartments in Vatican City and Castel Gondolfo, a small town near Rome.
The investigators also confiscated a gold nugget and a check made out to Pope Benedict XVI for 100,000 euros from the University Catolica San Antonio di Guadalupe and an original version of Virgil's Aeneid from 1581, from his apartment in Vatican City.
Clean shaven and dressed in a light gray suit, Gabriele appeared pale and largely expressionless during Saturday's session. None of his family attended.
Gabriele's lawyer, Christiana Arru, filed several motions concerning the admissibility of evidence, including the results of a psychological exam conducted without the presence of his lawyer and footage gathered via a hidden camera.
The court will hear from Gabriele himself in its next session, set for Tuesday next week.
Other witnesses called in his case include the pope's personal secretary, Georg Gaenswein, an attendant to the pope, Cristina Cernetti, and several police officers involved in the investigation.
Sciarpelletti was not present in court but was represented by his lawyer, Gianluca Benedetti, who declared his client's innocence.
Benedetti said Sciarpelletti was not a close friend of Gabriele, to which the latter reportedly nodded his head in agreement.
The presiding judge, Dalla Torre, agreed to a request from Benedetti that the two trials be separated.
The Vatican has previously said Gabriele cooperated with investigators and admits leaking the papers, which consisted of faxes, letters and memos, including some from a high-ranking church official expressing concerns about corruption within the Vatican.
A prosecutor in the case said in a report last month that Gabriele acted out of a desire to combat "evil and corruption everywhere in the Church."
"I was certain that a shock ... would have been healthy to bring the church back onto the right track," Gabriele is quoted as saying by the prosecutor, Nicola Piccardi.
The Vatican City State penal code for proceedings involving its citizens is based on the Italian penal code of the second half of the 19th century. Dalla Torre will lead the debate in the courthouse, located behind St. Peter's Basilica, and question the defendant directly.
Prison terms handed down by the court are served in the Italian prison system, under an agreement between the Vatican City State and Italy.
Gabriele was arrested in May, following a top-level Vatican investigation into how the pope's private documents appeared in the best-selling book "Sua Santita" ("His Holiness"), by Italian journalist Gianluigi Nuzzi.
The Vatican called the publication of his book "criminal" when it was released in Italian.