Chinese authorities have cremated the body of a revered Tibetan monk against his family's wishes, rights groups said Thursday, after protests erupted following news of his death.

Tenzin Delek Rinpoche's body has been cremated, Students for a Free Tibet (SFT) confirmed to AFP, citing relatives in India.

"Today, Tenzin Delek Rinpoche's body was cremated at a secret prison, located approximately 5km from Chuandong prison in Chengdu, at around 7am local Beijing time by the prison authorities as reported by his family members," the group said in a statement.

"The cremation took place against the wishes of his family to release the body in order to perform the final Buddhist rites in his hometown Lithang."

About 30 relatives and students were allowed to view the body before he was cremated and his family described the prison conditions as "worthy for a beggar", according to SFT.

They performed a short prayer and all of his belongings were burned, the group said.

Several protests in Delek's hometown and in the southwestern city of Chengdu, where he was jailed, have taken place since his death was announced, according to the US-based International Campaign for Tibet (ICT) and other Tibetan rights groups.

Police opened fire when around 1,000 people gathered near government offices in Nyagchuka county this week to protest the death, injuring about 20 people, the British-based Free Tibet group said.

The US-funded Radio Free Asia also reported that police fired on protesters, who were angry at a government decision to cremate his remains, rather than give him a traditional Tibetan burial.

Delek, 65, was 13 years into a life sentence for terrorism and separatism, imposed following a trial observers said was deeply flawed, when authorities announced news of his death at the weekend.

He was convicted of separatism and being involved in a bombing in a public square, but his supporters insisted he was innocent.

- Extremely angry -

Rights groups said the circumstances of his death remained unclear.

In its statement, SFT quoted Geshe Nyima, Delek's student and cousin, as saying:

"I am heartbroken, but also extremely angry. My family cannot accept this. We will not stop demanding justice until we have answers.

"Chinese authorities denied us our right to pay our final respect to Tenzin Delek Rinpoche and refused to return his body to us for Buddhist rites."

Chinese security forces had also briefly detained two of his sisters for not signing his health record, rights groups said.

Police held the sisters for "around 10 hours" on Wednesday, the ICT cited local sources as saying.

AFP was unable to independently verify the reports of the protests, or contact authorities in charge of Delek's case.

Access to Tibetan regions is tightly controlled by the Chinese government and local media is barred from reporting issues the government deems sensitive.

China, which has ruled Tibet since 1951, has been accused of trying to wipe out its Buddhist-based culture through political and religious repression and large-scale immigration by Han Chinese, the country's ethnic majority.

China says Tibetans enjoy extensive freedoms and that it has brought economic growth to the region.

Tibetan spiritual leader the Dalai Lama fled Tibet after an abortive uprising in 1959 and established his government-in-exile in Dharamsala, India.

Delek became one of China's highest profile Tibetan prisoners when he was sentenced, which the United States and the European Union at the time condemned. Washington said it was "saddened" by his death.

Hollywood star and Tibetan rights activist Richard Gere expressed outrage over Delek's death on Tuesday.