A Chinese rights activist who died under police guard in hospital had "black and purple" marks all over her body, her lawyer said Thursday, citing her brother.

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.

"The body looked horrible, it had been tortured, like it's not a human," Cao was quoted 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 (215 square feet), 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.

- Tragic example -

A spokesman for China's Foreign Ministry said Monday that Cao had suffered from "prolonged illness" and that she had suffered "multiple organ failure caused by tuberculosis and severe pneumonia despite all the rescue efforts".

"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".

Cao was a prominent human rights lawyer who had campaigned since 2008 for greater government transparency, and improved access for Chinese civil society to give evidence to a periodic UN review of China's rights record.

She had been set to travel to Switzerland last September to observe a meeting about China at the UN Human Rights Council, but Chinese authorities prevented her from boarding the flight.

She was initially held incommunicado, according to the UN expert group.

Her whereabouts became known when she was charged with the "crime of provocation", they said.

Countries including the US, Britain, France and Canada have also expressed concern at Cao's death.

Chinese dissident Li Wangyang, who was jailed for over 22 years for his role in the 1989 Tiananmen democracy protests, died in suspicious circumstances when he was found hanged at his hospital in June 2012.

Activists have rejected the findings of an official probe by Hunan provincial authorities -- where Li's hospital was located -- that said the nearly blind and deaf activist had committed suicide.