“Light off: rape.” To make the point, the Austrian prosecutor Christiane Burkheiser threw the switch in the courtroom. “Light on. Josef Fritzl leaves the cellar.”

The trial of Fritzl — on charges of imprisoning and raping his daughter for almost a quarter of a century and murdering one of the seven children he fathered with her — was never going to be routine. It was indeed billed as Austria’s Trial of the Century.

But no one had reckoned with the concentrated drama that was packed into the first day of hearings — from the moment Fritzl appeared with his head buried in a blue folder to shield him from the cameras, to the prosecution’s attempts to re-create the atmosphere of the dungeon, to the claims by his defence that he was no monster but a caring family man.

Ms Burkheiser’s task was to make the jurors feel like temporary prisoners of Fritzl. First, she had the height of the adapted cellar — he had told the authorities he was building a nuclear bunker — marked on the doorpost of the courtroom in St Pölten: 1.78 metres. Then, she passed to the jury objects retrieved from the cellar where Elisabeth gave birth to her children. “Smell!” said the prosecutor. And the jurors retched at the sheer pungency of 24-year-old mould.

“They had to crawl on their knees in order to get around the dungeon,” she told the court. “It was damp and mouldy. The dampness crept into their backs and into their bones.”

To drive home her point, the prosecutor threaded the phrase “light on, light off” throughout her speech.

Fritzl, she made clear, wanted absolute control over his downstairs family.

Was Ms Burkheiser’s performance enough to transport the eight jurors underground? Or to discomfit Fritzl? The 73-year-old building engineer had entered the courtroom hiding his face behind the large folder, his hands quivering, besieged by questions from the waiting media.

“How do you explain yourself? How can you abuse your daughter for 24 years?” asked Austrian television reporters, thrusting their microphones into the dock. He did not answer. Fritzl did eventually speak, after the cameras were excluded from the courtroom. Only then was the face exposed.

The voice, confirming his identity and marital status, was reedy, had none of the deep timbre that had been associated with the man who fathered 13 children, seven of them in an incestuous relationship with his imprisoned daughter. And the face — so tanned when he abandoned his entombed family for a month to take a holiday in Thailand — was parchment yellow, a prison complexion.

He had pleaded guilty or, as he said, partially guilty to rape — the prosecutor calculated that he raped his daughter 3,000 times — to coercion, to forced imprisonment, to incest. Not guilty to murdering one of his daughter’s babies — neglected so badly that he died, blue in the face, two days after birth — and not guilty to enslavement.

The pleas had been widely expected as had the essence of his defence, that he was no monster but rather a frustrated, violent father who cared for his so-called second family. “The exceptional aspect of this case is that my client did not behave like a monster,” said Rupert Mayer, his defence lawyer. “The special point is that he chose his daughter for this role, that he created children together with her and looked after them . . . he bought them schoolbooks, celebrated Christmas and birthdays with them.” He appealed to the jury to be objective. “If you just want to have sex, you don’t have children,” Mr Mayer said. “As a monster, I’d kill all of them downstairs.”

For his part, Fritzl told the three judges and eight jurors that he, too, had suffered abuse as a child. “I had a very difficult childhood,” he said in a trembling voice. “My mother didn’t want me. She was 42 when she had me — she simply didn’t want a child and she treated me correspondingly. I was beaten.” At the age of 12, he said, he had made it clear to his mother that he would not tolerate being beaten any longer and would defend himself. “From that point on, I was Satan personified for her,” he said. She never showed him any affection and his father appeared only “rarely and sporadically”.

The defence team strategy is plain: if they can get their client off the hook on the murder charge (a possible 15-year jail term) and enslavement (up to 20 years), then the maximum time that Fritzl would have to serve would be for the rape charge.

Under Austrian law that is ten years’ imprisonment. With good behaviour and counting the year spent in pretrial arrest, Fritzl could hope to be free by the Christmas of 2016 — a triumph of sorts for Mr Mayer, who ranks as one of the most effective defence lawyers in the country.

The judge could avoid this politically embarrassing outcome — Austrian tabloids are already demanding a lifetime behind bars — if Fritzl were sent to a guarded psychiatric clinic for an indefinite period of observation before starting his jail sentence.

But the prosecutor, Ms Burkheiser, 33, seemed determined yesterday to secure the longest possible jail term. First she had to dismantle the myth of Fritzl as a caring man. There were, she stressed, no mitigating circumstances. So the narrative underpinning the prosecution case became a horror story, the tale of a vicious man who abused his daughter for years before he threw her into his extraordinary dungeon at the age of 18.

The first part of the video testimony of Elisabeth, now 43, was shown to the jurors and to Fritzl in the darkened courtroom. Little notes — scribbled on the back of shopping bills left behind in the cellar by her father — constituted a kind of secret diary by Elisabeth, a chronicle of abuse.

The genre of the horror story has two conventions, and the prosecutor made use of both. The first is the mythic power of the locked door. The horror story seeks to stir the reader’s imagination about what lies beyond the door. For the imprisoned Fritzl family, the door became a potent symbol. “The accused repeatedly threatened the family with death if they should attempt to escape and he claimed to have an automatic switch which would cause a massive electric shock and a release of gas if the outer doors to the cellar were touched,” Ms Burkheiser said. “The effect would be death within a few minutes. There were no attempts to flee.”

Once he was beyond the door to the cellar, Fritzl let himself loose, behaved in an animalistic way. No details of his sex practices were made public yesterday and a court spokesman explained that they were one of the reasons why proceedings had to be held in camera.

The other horror story convention is the existential fear, the sense that one can wake up in an ordinary way one morning and by the end of that day be plunged into a living hell. That, said the prosecutor, is what happened to Elisabeth Fritzl on August 28, 1984. Drugged with ether and entombed in a soundproofed chamber behind eight locked doors for the next quarter of a century, Elisabeth lived a sub-human existence. There she gave birth — Fritzl provided a pair of rusty scissors to cut the umbilical cord.

It is not on the charge sheet, but Fritzl stole his daughter’s life. She and her downstairs children lived without sunlight, without human contact. On a whim Fritzl would cut off electricity for several days.

At lunchtime the drama ended abruptly — the one carried out in the public’s view, at any rate. Austria’s strict laws on privacy and the protection of victims does not merely restrict reporters in what they can say or write; if the judge so requires, it keeps them out of the courtroom altogether. Even the court spokesman, delegated with briefing the media at 4pm, would barely divulge anything, saying it was all confidential.

None of Fritzl’s alleged victims was present in court for the opening of the trial. They are spending the week in a psychiatric clinic to escape the publicity surrounding the trial. Journalists and the public are expected to remain excluded for the duration of the trial. They will be readmitted for the verdict, which is expected by Friday.

The horror story has one gentler variant: the tale where the protagonist, after undergoing numerous tortures, wakes up and realises he has merely survived an unpleasant nightmare. This, the prosecutor made plain, was sadly not the case of the Fritzl family. They are out in the sunlight at last, but their nightmare continues.

Chief prosecutor takes her first big case

Christiane Burkheiser, 33, is the star of the trial. She is conducting her first important case since being made chief prosecutor — she was on duty when the Fritzl family were released from the bunker in April. She has brought a new style to the courtroom: to evoke the dank atmosphere of the Fritzl dungeon she gave foetid pieces of material to the jury and ordered them to smell the evidence. Her office is only 20 metres away from Fritzl’s cell.

Rupert Mayer, 61, Fritzl’s defence lawyer, is one of the most flamboyant figures in the Austrian legal profession. The dandy has defended neo-Nazis, contract killers and, in one case, the parents of a girl held prisoner in a wooden cage. Mr Mayer makes a point of defending causes for which he has no sympathy: despite having a severe dog hair allergy he tried to enter the Vienna regional parliament as president of the newly established Party of Austrian Dog Friends.

Judge Andrea Humer, 48, is an expert in trials in which the press and public are excluded. She has applied some of this experience, arguing that Fritzl’s daughter needs protection. Her most prominent case so far has been a gay sex scandal in the theological seminary in St Pölten. It is unusual for both the prosecutor and the judge to be women, but this case demands sensitive questioning of the main victim, Elisabeth Fritzl.