Paolo Gabriele held in cramped isolation with lights on 24 hours a day for more than fortnight after arrest, Vatican court hears

The pope's former butler has lifted the lid on a secret world behind the Vatican walls during a dramatic cross examination as he stood trial for stealing and leaking the pontiff's private letters.

In a surprise statement, Paolo Gabriele claimed innocence before three Vatican judges and accused Vatican police of mistreating him while in custody.

The 46-year-old father of three said that after his arrest in May he had been held for up to 20 days in isolation in a room so narrow he could not stretch out his arms, and where the lights were kept switched on 24 hours a day, damaging his vision.

He added that Vatican police had put him under psychological pressure, denying him pillows on his first night in custody.

Domenico Gianni, the head of the Vatican police, who attended the hearing, was left visibly blushing by the claim, which prompted the presiding judge, Giuseppe Dalla Torre, to order an inquiry.

After the hearing the city-state's police force issued a statement saying Gabriele had asked for the light to be left on, had been allowed unlimited meetings with family and friends, and had been given pillows. After 20 days of incarceration he had been moved to a new cell which was being redecorated at the time of his arrest, the statement added.

Gabriele faces four years for leaking letters containing allegations of kickbacks and corruption at the Vatican to Italian journalist Gianluigi Nuzzi, who released them in a bestselling book this year.

Although he confessed to police after they found piles of letters in his apartment inside the Vatican, Gabriele said in court he was innocent of the charge of aggravated theft. "I feel guilty of having betrayed the trust of the holy father, whom I love as a son would," said Gabriele, who is expected to receive a pardon from Benedict XVI.

The former butler did, however, describe how he brazenly took photocopies of sensitive letters in the office he shared with two papal secretaries. But he denied stealing a gold nugget donated to the pope and a €100,000 cheque made out to the pope, both of which were found in a shoe box in his apartment.

Gabriele has described himself to police as "an agent of the holy spirit", seeking to lift the lid on Vatican sleaze that "scandalised" him, and to defend the pope. "At times the pope asked questions about things he should have been informed about," he told the court.

The pope's personal secretary, Monsignor Georg Gänswein, said he started suspecting a close papal aide of being the source of the leaks when letters that had not left his office were published.

Searching Gabriele's apartment, the police found letters dating back to 2006. One officer giving evidence said some documents dealt with the Italian secret services and the masonry.

Gabriele denied he had been aided by accomplices, though he has told police he had been in contact with four senior Vatican figures when he was collecting documents, including a secretary, two cardinals and a bishop. A Vatican IT expert is due to stand trial for harbouring documents for him.

The trial continues on Wednesday and is expected to wrap up by the weekend before the pope convenes a synod of bishops in Rome on Sunday.