Ding Zilin, who founded the Tiananmen Mothers victims group after losing her son in the military crackdown on the 1989 Chinese student movement, is receiving treatment in a Beijing hospital after a regular check-up on Wednesday.
Ding, 80, was a retired associate professor of philosophy at Beijing's prestigious Renmin University when her 17-year-old son Jiang Jielan died in the crackdown.
She is the founder of the Tiananmen Mothers, a group for the relatives of those killed or maimed in the crackdown that has vowed never to give up the struggle for recognition of their loved ones' deaths at the hands of the ruling Chinese Communist Party.
Group spokeswoman You Weijie told Hong Kong's Ming Pao News that Ding is being kept in for observation after the check-up.
"Ding Zilin has had a heart condition for a long time, and has been taking medication for it," You was quoted as saying. "This was a regular check-up, and the doctor wanted her to stay in hospital for observation."
The Tiananmen Mothers regularly call on the government to respond to calls for a reappraisal of the crackdown, which Beijing says was a necessary move to suppress a "counterrevolutionary rebellion."
Like many other Tiananmen Mothers members, Ding has faced routine harassment from Chinese law enforcement, but the group has continued to call regularly on the ruling Chinese Communist Party to meet its key demands for compensation, a reappraisal of the political verdict, an investigation into the deaths and injuries, and consequences for those deemed responsible.
Beijing forbids any show of public mourning for those who died on the night of June 3-4, while high-ranking leaders, often pressed by foreign journalists, have repeated their view that the party's verdict of "political turmoil" is accurate, and that the debate is closed.
There is no definitive figure for the number of people who died when People's Liberation Army (PLA) troops entered the capital in tanks, raking bystanders and buildings alike with automatic weapons fire, although estimates range from the hundreds to the thousands.
The crackdown brought down late premier Zhao Ziyang, whose attitude to the students was judged to be too liberal, and sparked several years of arrests and compulsory political studies sessions the length and breadth of China.
Reported by RFA's Mandarin Service. Translated and edited by Luisetta Mudie.