Pu Wengqing, 85, has spent the last two years campaigning for the release of her son Huang Qi, a prominent Chinese activist

For the last two years, Pu Wenqing, 85, has spent most of her time writing articles about her son, Huang Qi, a long-time dissident who has been in prison for more than two years awaiting trial. She posts updates about Huang, the founder of China’s first known human rights monitoring website, on Wechat and in October she recorded a video, pleading for help, and posted it on YouTube.

Now, Granny Pu, as she is known to her friends, has disappeared and is likely under house arrest or in a black jail, according to friends and human rights advocates.

On 7 December, she took a 10-hour train to Beijing from her home in Sichuan province with the goal of appealing to China’s leaders to release her son, who has been charged with leaking state secrets, a vague offence often used for activists. On the train, police questioned Pu and her travel companion several times and searched their luggage. When they arrived in Beijing, they were followed and then intercepted.

“Seven or eight men in plainclothes surrounded us and grabbed Granny Pu by her arms. Granny was pushed onto ground. I was screaming and shouting, asking people passing by to help us,” said Wei Wenyuan, who accompanied Pu to Beijing.

Photos show Pu, wearing a bright yellow traffic vest and clutching a case to her chest, laying on the ground. Pu and Wei were taken to a police station where police eventually took Wei away. “That was the last time I saw Granny Pu,” Wei said.

Friends of her son Huang have been searching for his mother, checking Pu’s home and the hospital where she used to work as a doctor. Pu Fei, who worked with Huang, said security officials in Neijiang in Sichuan said she was in their custody and safe. He was not given a reason for her detention. Pu Fei believes she is under house arrest at a location other than her home.

“It doesn’t have to be her home. A hospital, a hotel, a farm, a vocational village – they can all be a detention venues,” he said. According to Pu Fei, she is not in good health. “We are not sure whether she is safe or not, so we are very worried.”

Pu, retired and living on her pension, has become more active over the last two years since her son Huang’s most recent arrest. An outspoken critic, Huang was previously detained in 2008 and served three years in prison after meeting with the parents of children who died in the 2008 Sichuan earthquake, deaths many believed were the result of shoddily-built government schools.

Facebook Twitter Pinterest Pu Wenqing arriving at Beijing station on the day she disappeared. Photograph: Pu Wenqing

In 1998, he founded the website 64 Tianwang, originally to track missing people, which later became a hub for monitoring human rights abuses.

Huang has a potentially fatal kidney condition and is likely to die in prison, according to Pu. He has been denied treatment since being imprisoned, according to his lawyers, who said Huang has been beaten, is subject to constant interrogation, and is forced to stand for four to six hours at a time.

He was to have a pre-trial hearing on 10 December, but it was cancelled, according to his lawyers, who say they were not given any more information.

Several Chinese dissidents have died after being denied medical treatment, including Nobel Laureate Liu Xiaobo who died last year of lung cancer while serving an 11-year prison sentence. Cao Shunli, who was detained after demonstrating for public participation in a UN review of China’s human rights record, died in 2014 after being denied treatment.

Pu Wenqing is the mother of jailed dissident Huang Qi Photograph: Pu Wenqing

China has defended its actions by saying it has pioneered “human rights development with Chinese characteristics.” During a recent UN review of China’s record in November, vice foreign minister Le Yucheng said: “No country shall dictate the definition of democracy and human rights.”

Human rights advocates say the state’s treatment of people like Pu tells a different story. “The ongoing disappearance of Huang Qi’s elderly mother shows what ‘human rights with Chinese characteristics’ really means,” said Frances Eve, a researcher at Chinese Human Rights Defenders.

“Police find a woman in her 80s to be such a threat that they disappear her … all to silence her campaigning for basic due process rights for her detained son.”

According to friends, Pu lives modestly, shopping for the cheapest vegetables at the market and opting out of needed cataract surgery because of the cost. When friends give her food, she gives it to petitioners, residents appealing to authorities for legal redress.

Pu worries about her son constantly and occasionally gets angry or sensitive when discussing his case. “She always apologises afterwards,” said Wei.

In the video posted in October, Pu said: “I am already 85 years old. I want to meet my son. If I could meet my son just once more in my life, I could die without regrets … I beg you to speak out for my son.”