Paolo Gabriele took the stand at his historic trial accused of stealing secret memos that revealed intrigue and fraud allegations and leaking them to a journalist, in what he described as a bid to battle "evil and corruption" within the Vatican.

"Concerning the accusation of aggravated theft, I declare myself innocent," Gabriele said in his first public comments since his sensational arrest in May.

"I feel guilty for having betrayed the trust put in me by the Holy Father, whom I love like a son," he said, referring to a son's love for his father.

Gabriele also alleged he was mistreated during his 53-day detention in a Vatican prison, claiming he was held for up to three weeks at the start of his incarceration in a tiny cell where the lights were on 24 hours a day.

The Vatican immediately ordered an investigation into the claims. A father of three who is now under house arrest, the 46-year-old faces up to four years in jail if he is convicted on charges of aggravated theft.

Although he has admitted leaking the documents, his confession is not legally considered definitive proof for a conviction because he could have lied to protect others and there is no chance of a plea bargain under Vatican law.

On Tuesday the ex-butler, dressed in a grey suit, told judges in the tiny state's 19th-century courtroom in an area that is strictly off limits to tourists, that he had been driven to act because he believed the pope was being "manipulated."

"Inevitably my role allowed me to see many situations from both sides from what the people thought and what the powers that be thought," he said.

"What really shocked me was when I would sit down for lunch with the Holy Father and sometimes the pope asked about things that he should have been informed on," he said, adding: "I was the layman closest to the Holy Father."

"It was then that I became firmly convinced of how easy it was to manipulate a person with such enormous powers," Gabriele told the court.

The trial will continue on Wednesday when the last four witnesses will testify, after which the prosecution and defence will present their final arguments.

Vatican watchers said a verdict could come as early as Saturday. Gabriele sounded confident and sometimes emotional as he spoke but he was repeatedly interrupted by judge Giuseppe Dalla Torre as he tried to recount details of his network of contacts in the Vatican and his personal motivations.

When his lawyer asked about incidents he thought the pope should be informed about, the judge said: "It's irrelevant. We have to stick to the charge."

Gabriele also told the court he had acted alone without any accomplices but added that he had many high-up Vatican contacts who confided in him including a bishop, two cardinals and the pope's former German housekeeper Ingrid Stampa.

He said one of the cardinals, Paolo Sardi, had been "a spiritual father". Widespread unease in the Vatican Gabriele also spoke of his first contact with Gianluigi Nuzzi, the Italian journalist to whom he is accused of having leaked the documents.

"The idea was to find someone I could confide in that I could let off steam with, to communicate my state of mind and my unease at a situation that had become unbearabale and widespread in the Vatican," he said.

He said he had copied the documents during office hours using a photocopier in the office of the pope's two secretaries where he too had a desk.

The pope's personal secretary Georg Gaenswein, a hugely influential figure in the Vatican, also testified at the trial wearing his long black cassock.

Gaenswein was clearly awkward in a court context, at one point even telling the judge "it's my first time" but his answers were clear and concise.

When Gaenswein entered and left the court, Gabriele got up in a sign of deference to the German monsignor who was his direct superior.

As he spoke, Gaenswein shot a glance at Gabriele who remained impassive. At the start of the trial on Saturday, Gabriele suffered a serious setback when judges declined to include in the trial a top secret report on the "Vatileaks" scandal compiled by a committee of cardinals appointed by the pope.

The first hearing also revealed that Vatican gendarmes had seized 82 boxes of material from Gabriele's service apartments in the Vatican and at the pope's summer residence and installed a camera on the landing of his Vatican flat.

Two gendarmes testified on Tuesday, saying they had also found a lot of material about freemasons and secret services in Gabriele's apartment, as well as a gold nugget in a shoe box one of the items he is accused of stealing.

Gabriele said he had no idea there was a gold nugget in his home and denied any knowledge of a 100,000-euro ($130,000) cheque addressed to the pope from a Spanish university dean that police said was found during the raid.