ST POELTEN, Austria (Reuters) - An Austrian man who fathered seven children with a daughter he held in a cellar for 24 years pleaded guilty to incest on Monday but denied murdering their newborn son or enslaving her.

Wearing a mismatched grey suit, Josef Fritzl, 73, entered court in St Poelten, near Vienna, flanked by six policemen and concealing his face behind a blue loose-leaf binder.

He later lowered the folder and with his back to the gallery, spoke softly, sometimes mumbling, in response to questions about personal details and his plea.

Fritzl looked straight ahead or at his lawyer, but not at the jury or prosecutor, who accused him of raping his daughter in front of captive children.

Prosecutors said Fritzl, who faces life in prison if convicted, was responsible for the death of a twin who died shortly after being born in the cellar in 1996.

They said this was murder by neglect because Fritzl failed to seek help for the baby, whose body he burned in a furnace.

Prompted by the judge several times, Fritzl pleaded partially guilty to rape and said he was guilty of depriving the children who were kept underground of their liberty.

Austrian law differentiates between the severity of rapes and levels of coercion, and takes into account the degree of violence used and the consequences for the victim.

Fritzl's lawyer will clarify which part of the rape charges he rejects during the closed-door part of the trial this week, said court spokesman Franz Cutka. In theory, Cutka said, Fritzl could plead guilty to the rape charge in general but dispute the prosecution's claims as to the degree of violence he used.

Fritzl also pleaded innocent to a charge of enslaving his daughter Elisabeth for most of her life.

He shut (Elisabeth) away in the cellar and made her totally dependent on him, forcing her into sexual acts and treating her as if she was his own property, his charge sheet read.

A verdict is expected by Friday.

SOUNDPROOFED CELLAR

Fritzl, a retired engineer, built the soundproofed cellar with a reinforced door under his home in the town of Amstetten.

In an opening statement, prosecutor Christiane Burkheiser said that during the early years of her captivity, Elisabeth had no hot water, shower or heating.

It was sometimes so hot that condensation dripped down the walls, she told the jury, indicating the size of the 11-square-meter living space with her hands. Burkheiser said among the worst aspects of Elisabeth's ordeal was the uncertainty as to when he (Fritzl) would come down again, and rape before the eyes of the children.

Defense lawyer Rudolf Mayer said the charge of enslavement was inappropriate. He described how Fritzl brought up a second family and criticized his media portrayal as a monster, urging the jury to set emotions aside to enable a fair trial. He said Fritzl showed he was concerned for the children's welfare by taking some of them out of the cellar and his seriously ill daughter, Kerstin, 19, to hospital.

Kerstin had never seen daylight and her hospital trip led to Fritzl's arrest in April last year.

If he is convicted of murder by the eight-person jury, he could receive a life sentence or up 15 years in prison. Mayer said Fritzl expected to spend the rest of his life in jail, probably a closed mental institution.

Fritzl's daughter and her six children, three of whom were incarcerated from birth, are now living in a secret location under new identities.

Three of the children were raised above ground by Fritzl and his wife Rosemarie after he pretended Elisabeth had abandoned them. Police say Rosemarie did not know of her husband's actions.

In comments via Mayer last year, Fritzl said he had lived a second life in the windowless 60-square-meter (650 square foot) cellar complex, watching adventure videos with the children and bringing flowers for Elisabeth, who cooked dinner.