In his first interview since it emerged that Josef Fritzl imprisoned his daughter Elisabeth in a cellar beneath his home in August 1984 and forced to her to suffer a nightmare of incest, multiple rape and untreated births that lasted 24 years, the 73-year-old explained from his prison cell that, after the kidnapping, he saw no way out because he had become caught up in a "vicious circle" from which he could not escape.

He confessed to the lies he told police to keep his daughter hidden and spoke of the fears he had about being arrested, and of the hundreds of kilometres he drove each week to buy food for his cellar family to avoid detection.

But he also revealed the grim details of the life his daughter and three of their children – now aged five, 18 and 19 – were forced to endure in a windowless cellar without daylight or proper medical attention for so long. He recalled how his daughter, now 42, coped with losing all her teeth after more than two decades without dental care.

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"I grew up in Nazi times and that meant respect for authority and the need to control. I suppose I took on some of these old values," Fritzl said in the interview, published in the Austrian media. He recalled being brought up by his mother in a tiny flat. She had kicked his father out because he was a "waster". "I was totally in awe of her. Completely and totally in awe," he insisted.

Rudolf Mayer, Fritzl's lawyer, who has defended neo-Nazis during his career, conducted the interview which he said his client had given to counter what Fritzl had called the media's one-sided portrayal of him as a "monster". He believes Fritzl should be committed to a closed psychiatric unit for his crimes and not go to prison.

Fritzl was arrested 10 days ago and confessed to keeping his daughter incarcerated in a windowless cellar bunker beneath his home in the Austrian provincial town of Amstetten for 24 years.

In his interview, Fritzl denied allegations that he had threatened to gas his cellar family if they tried to escape, but he added: "I am sorry to say that I told them they would never get past the door because they would be electrocuted and die if they tried."

Elisabeth Fritzl is reported to have told police her father began sexually abusing her at an early age and that she was raped for the first time at 11. However, Fritzl claimed in his interview yesterday: "That is not true. I am not a man who has sex with little children. I only had sex with her much later, when she had been in the cellar for a long time."

He insisted his initial aim had been to "rescue" his daughter. "Ever since she started puberty, Elisabeth stopped doing what she was told, she just did not obey my rules any more. She would spend all night in bars and come back stinking of alcohol and smoke," he said. "I tried to rescue her from this swamp. I organised her a job as a waitress, but sometimes there were days when she would not go to work."

Fritzl said his daughter had tried to run away from home twice and hung around with "persons of questionable moral standards". He insisted: "That is why I had to arrange a place where I gave her the chance – by force – to keep away from the bad influences outside."

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The former electrical engineer, who was convicted of rape in the 1960s, incarcerated his daughter on 28 August, 1984. Her underground ordeal was to last 8,516 days. "I guess it must have been around 1981 or 1982 when I began to build a room in my cellar as the cell for her," he said yesterday, adding that he equipped it with a heavy concrete and steel, electrically operated door with a remote-control combination lock. "I then plastered the walls, added a washbasin and a small toilet, a bed and an electric ring, a fridge, electricity and lights," he said.

Fritzl said his desire to have sex with his daughter increased to a point where he was no longer able to control himself. He claimed that he raped her for the first time some eight months after her incarceration but denied that he had ever handcuffed her or tied her to a leash as has been reported.

"This was unnecessary. My daughter had no chance of escape anyway," he said. "At some stage at some time in the night, I went into the cellar and laid her on the bed and had sex with her. I knew that Elisabeth didn't want what I did with her but the pressure to do the forbidden thing was just too big to withstand. It was an obsession with me."

Fritzl admitted forcing his daughter to write letters to his wife Rosemarie which pretended that she had run away to join a religious sect. "With every week that I kept my daughter prisoner, my situation became more crazy. I often thought about telling a friend, but I was scared of being arrested. I got myself into a vicious circle from which there was no escape," he said. "I just kept putting off a decision."

In 1988, Elisabeth fell pregnant for the first time with their daughter Kerstin. Fritzl explained how he tried to calm his daughter down by giving her medical books about childbirth and providing her with towels and nappies. "It was great for me to have a second proper family in the cellar, with a wife and a few children," Fritzl said.

Indeed, a photograph of the family looks like that of any normal group of relatives. A teenaged girl in a checked blouse sits on the steps of her parent's rooftop swimming pool on a balmy summer evening in 1984. The sun has caught her bobbed red hair and a smile is starting to emerge on her lips. Yet the photograph, published in the German newspaper Bild, is no ordinary picture. It is the last image taken of Elisabeth before she was imprisoned.

Elisabeth bore seven children as a result of her incestuous relationship with her father, all without medical assistance. Yesterday, "Sissi's" last photo emerged together with a batch of letters written in teenage schoolgirl handwriting to a penpal, showing that only weeks before she disappeared, she had not only acquired a boyfriend but had well developed plans to leave home for good.

They coincided with the publication of an extraordinary interview with her father Josef, in which he attempted to justify the crimes to which he has confessed, claiming that his urge to rape his daughter had turned into an "obsession" which became too powerful for him to control.

Elisabeth's letters, sent to her penfriend, showed that she planned to leave home and move in with her sister as soon as possible. At the time, Sissi was training to be a caterer and had already started work as a trainee at a motorway restaurant. In the summer of 1984, she wrote: "After I have done my tests, I am going to move in with my sister and her boyfriend. They can't afford the flat by themselves. It's a bargain for me and I've got two rooms just for myself."

The letter explains that Sissi has a steady boyfriend who is doing training with her, but has joined another course to learn about cooking and being a waiter. She complains that he lives far away in another town. "That's why I am so sad."

Her letters paint a picture of a happy young woman who plays tennis, goes swimming and even plays football on her two free days a week. But the letters also give dark hints that she is already being abused by her father. "Sometimes I get pains and I feel sick again," she writes. "I am supposed to be off sick at the moment, but I am completely stressed out. My nerves are not in good shape either." The photograph is signed in Elisabeth's own handwriting with the words "Think of me !!! - Sissi".

Their son Stefan was born in 1990. But as the other children were born from their incestuous relationship, Fritzl hatched plans to move them upstairs when they were still babies. Again, he forced his daughter to write letters begging her mother to adopt the children because she could not cope.

Fritzl's Jekyll and Hyde double life went on for more than 20 years. Upstairs, he and Rosemarie kept their official family. She apparently never suspected anything and never complained about his sex holidays in Thailand. In the cellar below his home, Fritzl kept his "second wife" as he called his daughter, bringing her flowers and their children books and cuddly toys.

In 1993, he extended the bunker and installed a television, video recorder and radio in the cellar. He added: "I even gave Elisabeth a washing machine in 2002, so that she wouldn't have to wash all the clothes by hand.

"It just became a matter of course that I lived my second life in the cellar. We celebrated birthdays and Christmas down there. I even smuggled a Christmas tree into the cellar with cakes and presents."

But Fritzl also admitted that decades underground without daylight, or medical and dental care, severely affected his cellar prisoners. "Elisabeth stayed strong, she never complained, even when her teeth slowly went rotten and fell out of her mouth one by one. She suffered day and night with unbearable pain and could not sleep," he said.

In an attempt to cure his cellar family's illnesses, which included flu, coughing attacks and strange fits suffered by Kerstin and her five year-old brother Felix, Fritzl gave them aspirins. However the two children appeared to have inherited an allergy to the drug from their grandmother. The boy suffered fits of shaking and Kerstin would scream uncontrollably.

Fritzl said his daughter finally arranged a plan of action with him, and wrote a letter that was designed to accompany Kerstin, who, by that time, had lapsed into a coma and urgently needed hospital treatment. The letter eventually became the second family's passport to freedom. Kerstin was taken to hospital, Elisabeth was allowed to join her and contacted the police.

However, Fritzl maintained that it had been his intention all along to free his cellar prisoners and "bring them back home" as he put it. "I was getting older and I knew that, in future, I would no longer be able to care for my second family in the cellar. The plan was that Elisabeth and the children would explain that they were kept in a secret place by a sect. They betrayed me rather sooner than I expected as the problem with Kerstin escalated," he said.

Fritzl is to appear in court today and judges will almost certainly rule that he remain in prison until his trial. Fritzl said in his interview that he understood the "anger" people outside felt about his crimes and said inmates in his prison in the Austrian city of St Pölten banged on the walls of his cell and shouted: "Satan we are going to kill you" at night. "I only want one thing now," he insisted, "and that is to pay for what I did".