US-based Human Rights Watch urged China to immediately lift all restrictions on "Singing Nun" Phuntsog Nyidron, who was released after being Tibet's longest serving female political prisoner.
Phuntsog was released in March after 15 years in Tibet's notorious Drapchi Prison but has been under constant government supervision since then, Human Right Watch said in a statement.
"At least two security officials from a pool of four are with her 24 hours a day," it said. "Anyone coming to her home must sign in."
Phuntsog, 38, was arrested during a peaceful demonstration in Lhasa in October 1989, days after the exiled Tibetan spiritual leader the Dalai Lama was awarded the Nobel Peace Prize for his efforts to negotiate greater autonomy for the Himalayan region with China.
Picked out as the ring-leader of the protests, Phuntsog was sentenced to nine years in prison.
Her sentence was extended in 1995 after she and a group of 13 other jailed nuns recorded Tibetan spiritual songs from their prison cells, which eventually were widely disseminated in Tibet, much to the displeasure of the Chinese government.
"China tries to score points with other governments by opportunistically releasing activists, then keeping them isolated and under constant surveillance," said Brad Adams, executive director of New York-based Human Rights Watch's Asia Division.
"This is a nasty game China plays, and the international community must publicly condemn it."
Human Rights Watch also expressed concern about Phuntsog's medical condition.
It alleged that during her years in Drapchi prison, she was subject to severe beatings.
"Sources familiar with her have reported kidney damage attributable to the beatings and memory problems from repeatedly being hit on the head," it said.