Authorities in central China have agreed to perform an autopsy on Chinese dissident Li Wangyang, who died this week in allegedly suspicious circumstances, his family said Thursday.

Li, 62, spent 22 years in jail for his role in the Tiananmen democracy protests and was found dead in his hospital ward in central China's Hunan province on Wednesday by his sister and brother-in-law.

"They have not cremated him yet. The authorities have agreed to an autopsy," Li's brother-in-law Zhao Baozhu told AFP.

"As we don't understand these things, they said we could have a lawyer or an expert attend the autopsy."

Zhao said he and his wife were under police supervision in a Shaoyang city hotel, but denied they had been placed under criminal detention.

"Police have told Li Wangyang's friends and supporters to go home and not cause trouble, they won't allow them to visit us," Zhao said.

"We have demanded that they allow us to hold a funeral, we hope friends and supporters will be allowed to attend."

Zhao refused to discuss the possibility that Li was beaten to death by security guards, a scenario put forward by Li's supporters and numerous rights groups.

The activist had been in good spirits on Tuesday night and last week told Hong Kong journalists he was prepared to continue to fight for democracy in China, friends said.

According to the New York-based Human Rights in China (HRIC), Zhou and Li's sister, Li Wangling, found the dissident dead on Wednesday morning strung up to a ward windowsill by a bandage wrapped around his neck, with both his feet on the ground.

Li was under round-the-clock police surveillance in the hospital at the time, the group said.

The activist was sentenced to 13 years in prison for "counter-revolutionary" crimes for organising workers in Shaoyang into an autonomous union during the 1989 Tiananmen democracy protests, which were centred in Beijing, HRIC said.

He served 11 years and was released, but was given another 10 years' jail in 2001 for "inciting subversion" after he tried to sue the authorities over prison mistreatment that left him nearly blind, almost deaf and with numerous other health problems.

Police in Shaoyang did not answer phone calls on Thursday.

By Thursday afternoon, more than 3,000 people from China and around the world had signed an online petition calling on China's government to launch a public investigation into Li's death.