Josef Fritzl used his daughter as a "toy", an Austrian has court heard, after the 73-year-old admitted imprisoning her in an underground bunker for 24 years and forcing her to bear seven children.
Read more: Fritzl tells the court he was abused as a child
But the Austrian engineer, who set up a house of horror with electrically controlled doors to stop Elisabeth Fritzl and her children from escaping, denied slavery and a murder charge over the death of one of the incest babies.
PHOTOS: Fritzl's trial begins
The white-haired Fritzl, dressed in a grey suit, hid his face behind a blue document folder as he entered the court.
It was his first appearance in public since the shocking case broke last April when the eldest child, Kerstin, 19, who had lived her entire life underground with two brothers and her mother, fell severely ill and had to be hospitalised.
Fritzl is accused of holding his daughter Elisabeth captive for nearly a quarter of a century and fathering seven children with her.
Fritzl admitted rape, incest, sequestration and coercion, replying with a simple "yes" to the question as to whether he pleaded guilty when the charges were read.
For these, he faces a prison term of up to 15 years.
But he pleaded not guilty to a murder charge which carries a life sentence. Prosecutors accuse him of letting one of the babies die shortly after birth in 1996, when he failed to seek medical aid after birth.
Fritzl told police the baby was stillborn and he burnt the body in a boiler in the cellar.
He likewise pleaded not guilty to a charge of enslavement the first time such a charge has been brought in Austria as the trial started in Sankt Poelten, some 60km from the family home in Amstetten.
He also denied telling Elisabeth and the children they would be gassed if they tried to escape, which, according to the 27-page charge sheet, constitutes grievous assault.
"No, I didn't say that," Fritzl said.
At the start of the trial, prosecutor Christiane Burkheiser said Fritzl used his daughter "as a toy" during the 24 years he held her captive in a narrow cellar with no hot water, no heating, no fresh air or sunlight, describing his crimes as "inconceivable."
"He came (to the cellar). Lights out. Rape. Lights on," she said.
"I went down there twice and there's a morbid atmosphere," added Burkheiser.
"It's damp, it's musty, it's mouldy."
Fritzl's lawyer Rudolf Mayer, who has argued his client was only trying to create a second family, maintained that he was no monster.
"If you only lock up your daughter to have sex with her, you're not going to have any children, you're not going to get schoolbooks (for them) or a Christmas tree."
Fritzl "had feelings of guilt for 24 years," he added, citing from the psychiatric expertise.
Burkheiser noted however that Fritzl had raped his daughter in front of her own children and the defendant "showed no sign of regret or any consciousness of wrongdoing."
Three of the children had been brought to live with Fritzl and his wife, but the three other surviving ones spent their entire lives in the dungeon, never seeing daylight until their release last April.
Authorities are trying to organise a new life for the woman and her children under a new identity. She has returned to a special clinic for the duration of the trial to avoid any possibility of publicity.
After the reading of the charges and the opening statements by the prosecution and the defence, the public and media were led out of court and proceedings continued behind closed doors.
Court spokesman Franz Cutka said the court began hearing some of Elisabeth's videotaped testimony.
Only she will testify, with all other family members declining to testify as permitted under Austrian legislation.
There will also be four expert witnesses: a psychiatrist, a technical expert, a neo-natal expert who must help determine whether Fritzl let a baby die shortly after birth and an electro-mechanical expert, who will testify about the complex locks Fritzl installed on the doors to the cellar.
Cutka said that, if the proceedings moved quickly, "it's possible we'll get to the verdict by Thursday," instead of Friday, as initially planned.
Fritzl's wife, Rosemarie, 69, told British newspaper The Sun that she had changed her name and moved to a new town in an attempt to disguise her links to Fritzl.
"My life has already been ruined enough. I don't have any money. My pride is all I have left, that and my family. All I want is to keep my family," she said.
The report said she rarely leaves her flat, has her groceries delivered to avoid having to venture outside and only admits visitors after screening them through an intercom.
"I just want to be left alone," she said.