Josef Fritzl hid his face behind a blue file folder as a judge began the proceedings under heavy security in St. Poelten, 65 kilometers (40 miles) west of Vienna. Fritzl pleaded guilty to incest and false imprisonment, but only partially guilty to charges of coercion and rape. He pleaded not guilty to murder and enslavement.
Fritzl, 73, faces up to life imprisonment if convicted of murder in Austria, which has no death penalty. A verdict was expected by Friday.
If convicted of enslavement, Fritzl could face up to 20 years behind bars. For rape, he could get up to 15. Incest is punishable by up to one year in prison.
Authorities say Fritzl imprisoned and repeatedly raped his daughter, Elisabeth, for 24 years in a cramped and windowless dungeon he built beneath the family's home in the western town of Amstetten. Investigators say DNA tests show he fathered her six surviving children.
The crime stunned people worldwide when it came to light last April.
Another child died in infancy, and that prompted the murder charge. Prosecutors contend the baby boy might have survived if Fritzl had arranged for medical care.
In her opening statement, prosecutor Christiane Burkheiser accused Fritzl of repeatedly raping his daughter in front of the children.
Burkheiser said Fritzl didn't talk to his daughter during her first few years in captivity and that he simply came down to the cellar to rape her.
"Josef Fritzl used his daughter like his property," Burkheiser said.
She alleged that Fritzl once punished the young woman by shutting off electricity to the dungeon, and forced her to spend the first part of her captivity in a tiny space that didn't even have a shower or warm water.
"The worst was ... there was no daylight," Burkheiser said, adding it was also "incredibly humid" in the cramped space and the air was moldy and stale.
Burkheiser said Elisabeth was "broken" by Fritzl's alleged actions and the uncertainty of her fate and that of her children.
"He was the one who decided when to bring groceries ... there were shortages," she said.
Defense lawyer Rudolf Mayer appealed to the jury to be objective and insisted Fritzl was "not a monster," saying his client even brought a Christmas tree down to his captives.
"If you just want to have sex, you don't have children," Mayer said. "As a monster, I'd kill all of them downstairs."
Fritzl, who wore a mismatched suit, spoke in an almost inaudible voice as he gave the judge his name and other personal details.
He eventually removed the folder from his face, but sat still in the dock, clasping his hands together, and stared straight ahead. He answered several questions from the presiding judge, but it was unclear exactly what he said.
Mayer said he had no particular strategy for Fritzl's defense.
Before the trial got under way, Mayer told The Associated Press his client was nervous. "He told me, 'I'm scared, Mr. Mayer,'" the lawyer said.
Court spokesman Franz Cutka said Fritzl's partial admissions of guilt on the rape and coercion counts would become clearer over the course of the proceedings. Cutka suggested it might be a defense tactic to lessen Fritzl's sentence.
Security was tight in St. Poelten. Police imposed a no-fly zone above the courthouse to dissuade reporters from renting helicopters for aerial shots - and to prevent prison breaks from the jail next door where Fritzl has been in pretrial detention.
Mayer welcomed the security, saying both he and Fritzl had received threats.
Three of the children grew up underground in Amstetten, never seeing daylight. The other three were brought upstairs to be raised by Fritzl and his wife, Rosemarie, who apparently believed they had been abandoned.
The children, together with Elisabeth, initially recovered from their ordeal in a psychiatric clinic and then were moved to a secret location. To ensure their security and privacy during the trial, they have since returned to the clinic, where guards are on high alert.
The Associated Press normally withholds the names of victims of sexual assault. In this case, withholding Elisabeth's name by the AP became impractical when her name and her father's were announced publicly by police and details about them became the subject of massive publicity both in their home country and around the world.
None of the victims is expected to testify in court. Instead, the eight-member jury will see prerecorded video testimony from Elisabeth and from one of her brothers, Harald.
Associated Press Writer William J. Kole in Vienna contributed to this report.