The pope's former butler Paolo Gabriele, 46, smiled and joked on Saturday as his trial for stealing Benedict XVI's most private correspondence got under way at the Vatican.

The man who claims he was working as an "agent of the Holy Spirit" when he leaked letters exposing corruption allegations at the Holy See appeared unfazed as one investigator revealed in court that evidence included 82 cardboard boxes of material seized from him.

Gabriele was arrested in May after Vatican police found piles of letters and papal gifts he confessed to stealing from Pope Benedict's desk, including a cheque for €100,000 (£79,000) donated by a Spanish university, a gold nugget and a 16th-century copy of Virgil's epic poem the Aeneid.

The trial is unprecedented for the Vatican, which jealously guards its secrets. No cameras were allowed in, while pens were confiscated from the eight journalists admitted in case they concealed recording devices. Once inside they were handed new pens.

A defence request to admit evidence from a separate inquiry into the scandal by a team of cardinals, which was commissioned by the pope, was turned down.

The court's three judges are set to call a series of witnesses, including Monsignor Georg Gänswein, the pope's personal secretary, who maintains a low profile but is thought to wield great influence at the Vatican. Cristina Cernetti, one of the lay nuns who cook and clean for Pope Benedict, was also called.

The court opted to try separately Claudio Sciarpelletti, a Vatican IT technician accused of handling sensitive documents on Gabriele's behalf and called William Kloter, the vice-commander of the Vatican's Swiss guard as a witness in his trial. Sciarpelletti declined to appear in court on Saturday because he was too "agitated", his lawyer said.

Dressed in a grey suit and tie, Gabriele looked on impassively as his lawyer, Cristiana Arru, argued that parts of his interrogation should be excluded from the trial since no lawyer had been present and requested that the gold nugget be fingerprinted, a request that was turned down.

Gabriele, who faces a four-year sentence for aggravated theft, has been depicted by experts as a paranoid individual who craves the attention of others and is vulnerable to manipulation – a judgment that could make a papal pardon more palatable.

After beginning to leak documents, Gabriele told investigators: "I reached the point of no return and could not control myself any more."

The pope has said Gabriele's arrest "brought sadness in my heart".

But investigators have made it clear that they are still searching for other Vatican staff they suspect leaked embarrassing documents to the Italian press.

The trial resumes on Tuesday.