The Hong Kong-based Information Center for Human Rights and Democracy in China said police beat Zhang Min and forced her to swallow salt water until her lungs filled.
She was the 186th Falun Gong member to have died at the hands of Chinese authorities since the movement was banned in July 1999.
Zhang was arrested after handing out Falun Gong leaflets. She died December 5 in the Number two Detention Center in Yilan in Heilongjiang province, more than 700 miles northeast of Beijing, said the center.
The Falun Gong says that more than 230 of its members have perished while in police custody. But Beijing maintains that Falun Gong practitioners have killed themselves either by following the teachings of leader Li Hongzhi and discarding prescription medicine or through suicide.
The rights group said that Yilan police detained Zhang and eight other followers as they handed out Falun Gong late last month. It said in attempts to investigate where the promotional material was being produced, Zhang was beaten. Two days after her death, Zhang's body was cremated.
The rights group said there was no further information about the eight other Falun Gong followers who were detained with Zhang.
On Sunday the official Chinese news agency Xinhua reported, "a murder recently uncovered by police has revealed more irrefutable evidence about the inhumanity of the notorious Falun Gong cult, headed by Li Hongzhi."
It said, "On November 25, 2001, a Falun Gong practitioner ruthlessly killed his father and wife, and badly injured his mother at his parents' home in Xicheng District of Beijing."
"After receiving a report at 17:55 that day, police immediately rushed to the scene and were horrified by what they saw: bloody floor, walls, doors and corridor, a woman (later identified as the killer's wife) with many knife wounds in her face and arms, and two elderly people (later identified as the killer's parents) lying in pools of blood," said Xinhua.
The alleged killer, Fu Yibin, was apprehended at the scene and confessed to the police that he had stabbed and killed his father and wife to death and had injured his mother after he realized that "they had to depart." Fu, Xinhua said, began practicing Falun Gong in 1998.
Fu told police "he killed his father and wife to save them from the " sea of bitterness," give them a "real life" and send them to a "land of happiness," said the agency.
After Beijing outlawed the movement, its propaganda machine worked at fever pitch. Testimonials of former Falun Gong members told of their bad experiences while under the spell of the Falun Gong and were broadcast on state-run television and radio and filled the pages of official newspapers.
The Falun Gong says it is a peaceful movement and promotes good health and longevity.