Lower Austria police said in a statement that the 42-year-old woman, identified as Elisabeth F., had been missing since Aug. 29, 1984. She was found by police in the town of Amstetten on Saturday evening following a tip.
Franz Polzer, head of the Lower Austrian Bureau of Criminal Affairs, told reporters that the 73-year-old father, identified in a separate police statement as Josef F., had been taken into custody.
In a chronology of events outlined in a statement, police said a letter written by Elisabeth F. had apparently surfaced a month after her disappearance asking her parents not to search for her.
Police said that, during questioning, Elisabeth F. told them her father began sexually abusing her when she was 11. Police said she alleged that he sedated, handcuffed and locked her in a room in the cellar on Aug. 28, 1984 in the Lower Austrian town of Amstetten.
During the 24 years that followed, she said she was continually abused by her father and gave birth to six children, the statement said. In 1996, she said she gave birth to twins but one died several days later because it was not appropriately cared for; police are investigating. Josef F. had then apparently removed the corpse from the cellar and burned it, the police statement said. It was not immediately clear if the twin who allegedly died was included in the police tally of the number of children.
On Sunday evening police said investigators found the area where the woman had been held captive along with three of her children.
In an interview with Austrian broadcaster ORF, Polzer said the area had "several" rooms and a "very narrow" hallway.
Police found it after Josef F. gave them a code to unlock a hidden door, Polzer said.
On its Web site, ORF reported that the rooms also were very narrow and at most 1.7 meters (5.6 feet) high.
Police picked up Elisabeth and her father on Saturday close to the Amstetten hospital after they received a tip.
According to the police statement, Josef F. had freed Elisabeth and two of her three children from the cellar, and had told his wife that she had come back to them. The third child in the cellar, Kerstin F., was found unconscious on April 19 in the apartment building where the grandparents live, along with a note from Elisabeth asking that she be taken care of. She was then hospitalized.
Police said Elisabeth F. appeared "greatly disturbed" psychologically during questioning. She agreed to talk only after authorities assured her that she would no longer have to have contact with her father and that her children would be cared for.
One of her children, identified as 19-year-old Kerstin F., is currently hospitalized in Amstetten in very serious condition.
Police said three of the children were registered with authorities and lived with the grandparents.
According to the police statement, Josef F. and his wife, Rosemarie, had told authorities they had found those children outside their home in 1993, 1994 and 1997, each time with a note from the mother. In the first letter, Elisabeth F. had apparently said she already had a daughter and son. In another letter, she said she gave birth to another son in December 2002, according to the statement.
The other three children were apparently held captive in the cellar with their mother, Polzer told reporters in broadcast remarks.
"Elisabeth F. taught them how to speak," Polzer was quoted as saying by the Austria Press Agency.
The agency quoted police as saying Josef F. has been arrested but had not confessed.
The agency also quoted Gerhard Sedlacek, a spokesman for the public prosecutor's office in St. Poelten, as saying that the surviving children three boys and three girls are aged between 5 and 20.
DNA tests are expected to determine whether Josef F. is the father of the children.
Sunday's developments are reminiscent of the case of Natascha Kampusch, which shocked Austrians less than two years ago.
Kampusch was 10 years old when she was kidnapped in Vienna on her way to school in March 1998. She was held for the next 8 1/2 years by Wolfgang Priklopil, who largely confined her to a tiny underground dungeon in his home in a quiet Vienna suburb. Priklopil threw himself in front of a train just hours after Kampusch's dramatic escape on Aug. 23, 2006.