* Butler says eyesight hurt, denied pillow after arrest
* Says leaked documents but not guilty of "aggravated theft"
* Feels remorse, guilt for betraying pope (Updates with quotes, colour)
VATICAN CITY, Oct 2 (Reuters) - Pope Benedict's former butler, on trial for stealing papal documents, told a Vatican court on Tuesday that during the first weeks of his detention he was held in an isolation room so small he couldn't stretch out his arms and with light on constantly.
Paolo Gabriele said that during those weeks he had suffered damage to his eyesight and had felt under psychological pressure. On the first night in the room in the Vatican's police station, "even a pillow was denied me", he said.
A judge ordered an investigation of the police force after Gabriele, speaking confidently and smiling often, made the assertions on the second day of a trial that has embarrassed the Vatican.
The leaks have laid bare the inner-workings of the tiny city-state at a time when the Vatican has been eager to clean up its image after a series of scandals involving widespread sexual abuse of minors and mismanagement at its bank.
Gabriele, who is accused of passing to a journalist documents alleging corruption in the Vatican, pleaded not guilty to charges of aggravated theft.
But he said in his testimony, the first time he has spoken publicly since his arrest, that he considered himself "guilty of betraying the trust of the Holy Father, who I loved like a son (loves a father)".
Wearing a smart grey suit, he said he acted because he could see a wide gap between the way ordinary people perceived things in the Church and the way they were seen by "those at the pinnacle of power".
Gabriele has not denied stealing copies of documents and leaking them but told investigators he did so because he saw "evil and corruption everywhere in the in the Church" and wanted to help the pope.
Asked by his lawyer Cristiana Arru if it was true that for the first weeks after his arrest on May 23 he was held in a room so narrow he could not stretch out his arms, he said: "Yes."
In answer to a question by the judge, Gabriele said:
"For the first 15-20 days the light was on 24 hours a day and there was no switch. As a result my eyesight was damaged."
He said he was subjected to what he and his lawyer called psychological pressure.
After hearing the accusations of abuse, the president of the three-judge panel, Giuseppe Dalla Torre, told Vatican prosecutor Nicola Picardi to open an investigation into the allegations.
Domenico Giani, the head of the Vatican police, issued a statement saying the room conformed to "standards used by other countries in similar situations".
It said the light had been kept on for general security reason, to keep Gabriele from harming himself and that the prisoner had been given an eye mask. He denied that Gabriele had not been given a pillow and said Gabriele was later moved to a larger room in the Vatican police station.
Monsignor Georg Ganswein, Benedict's private priest-secretary, appeared uncomfortable during his testimony as he answered questions about the daily routines of the papal household and recounted how he confronted Gabriele about the leaks.
Gabriele, who appeared calm during the course of the three-hour session, stood up in a spontaneous sign of respect when Ganswein, his former boss, walked in.
Ganswein, like the other witnesses, put his right hand on a book of the gospels and swore to tell the truth.
Gabriele earlier had suggested that important information had been withheld from the pope but did not say who had done so.
"At times the pope asked questions about things he should have been informed about," Gabriele said, adding: "I became convinced that it is easy to manipulate a person who has enormous decisions-making power in his hands."
He said he went to his "confessor" when he realised he was doing something wrong but did not stop and that he was never was offered money for the leaked documents.
The papers Gabriele admits he photocopied and passed on at secret meetings included letters to the pope in which a senior Vatican functionary expressed concern about improper behaviour in the Holy See's business dealings.
The letter-writer, Archbishop Carlo Maria Vigano, was later posted to Washington despite pleading to be allowed to remain at the papal state.
Gabriele said he did not have any direct accomplices but was influenced by others and by a widespread malaise in the Vatican.
Vatican policemen testified how they seized so much material from the Vatican apartment where Gabriele lived with his wife and three children and from his quarters at the pope's summer residence that they filled 82 boxes.
The trial was adjourned until Wednesday morning. (Editing by Michael Roddy)