Lawyer Wang Yu called for an independent investigation into the death of Cao Shunli, which sparked international condemnation.
Cao, who died last Friday, had been denied access to vital medical treatment for months, her brother and lawyers acting on her behalf earlier told AFP.
Wang, lawyer for the 52-year-old activist, told reporters in Hong Kong that Cao's body was disfigured, citing her brother who saw her corpse on the day she died.
"The body was covered with black and purple marks, the arms were scaly, the whole body was swollen," she quoted Cao Yunli as saying.
Hospital staff had been instructed not to let Wang see the corpse, she added.
Wang, who said she was "under pressure" not to talk publicly about the case, said Cao told her she had been detained in a room measuring 20 square metres, with as many as 18 others.
She had been moved to a hospital in February where she was under guard.
Wang called on mainland Chinese authorities to mount an independent investigation into the cause of Cao's death.
"I hope an independent party will investigate the cause of death because I don't believe Cao died of bacterial pneumonia," which was entered on the official death certificate, she said.
"During her illness she has received serious treatment and her lawful rights and interests have been protected in accordance with law," Hong Lei said.
Cao had been released on bail in February pending trial, Hong added, but Wang said she had not been allowed to visit her in hospital.
A group of UN rights experts said on Tuesday that Cao's death was "a tragic example of the results of criminalisation of the activities of human defenders in China and reprisals against them".