Relatives of a Tiananmen Square protest veteran have been detained by police after raising concerns that he died in strange circumstances, a rights group has said.

Li Wangyang was found dead in a hospital ward in Shaoyang, central Hunan province, on Wednesday, two days after the anniversary of the brutal crackdown on the 1989 pro-democracy protests. He had a noose around his neck and local officials said he had killed himself.

But thousands of people have signed an online petition calling for an investigation into his death after relatives said his feet were on the floor when he was found and questioned whether he was physically able to hang himself given his ill health.

Li, 62, had spent more than 22 years in jail for his role in the protests in Beijing, but was released last year. He was blind, had severe hearing difficulties and was being treated for poor health.

Human Rights in China said police had taken Li's sister Li Wangling and her husband Zhao Baozhu to a hotel and were holding them there, citing an unnamed source who said the couple felt isolated and feared for their safety.

Earlier this week Zhao told the Los Angeles Times: "[Li] could barely hold a bowl without his hands shaking … how could this happen when there were security guards watching him? We have many questions."

The source told the group police had agreed to delay the cremation of Li's body and allow the couple to watch the postmortem. But they have been unable to contact Guangzhou rights lawyer Tang Jingling since hiring him to represent them. Tang's current whereabouts are unknown, the group added.

"HRIC is deeply concerned about the safety of Li's sister and brother-in-law and urges that the authorities immediately restore their personal freedom and ensure the right of Li's family to retain legal representation of their own choosing," the group said in a statement. Calls to police in Shaoyang were not answered.

Li was jailed for instigating counter-revolutionary propaganda after co-founding the Labour Autonomous Union to support the student protestors in 1989. When he was released in 2000 he sued for compensation – earning himself another lengthy sentence for the same crime.

In his last interview, with Hong Kong cable television, Li described being tortured but said he had never regretted supporting the students. "Even if I were beheaded, I would not regret it. For democracy and the survival of the country, the ordinary man should take responsibility," he said.

"I believe in the future China will be on the path of freedom and democracy and the implementation of a multi-party system. The date is not so far away."