German police on Tuesday freed a mentally disabled 26-year-old woman who was believed to have been locked away in an apartment by her own mother, possibly for years.

The young woman was found in a "physically run-down and neglected state" in a rubbish-strewn flat in the southern city of Rosenheim, a police spokesman said, adding that she had been sent to a psychiatric hospital.

"Communicating with her is very, very difficult," the spokesman, Stefan Sonntag, told the DPA press agency.

Police discovered the captive by chance while carrying out an eviction order.

As they sought to enter the apartment, the 54-year-old tenant, who is believed to be the captive's mother, threw herself down the stairwell shaft from the second floor in an apparent suicide attempt, the spokesman said.

The woman suffered severe injuries from the fall and was taken to hospital.

When officers subsequently entered the apartment they broke down a locked door and discovered the younger woman.

"We believe that this is the tenant's daughter, who was in a physically run-down state and who had lived in there for a long time, maybe even for years," Sonntag told local news site rosenheim24.de.

An unidentified neighbour told the news site that he had lived next door since 2003 but had never seen the 26-year-old.

Another neighbour estimated that the young woman had been held inside for five years, after leaving a mental institution.

That neighbour said the young woman would cry out and knock on the walls at night.

"One time, last year, she managed to get out of the apartment. When we saw her we didn't think there was anything unusual going on," the neighbour told DPA, adding that the mother would refuse any offer of help.

German media, citing locals, said a younger brother, aged around 15, also lived in the house with the two women.