Runggye Adrak had called from a festival stage for Tibetan freedom and the Dalai Lama's return.
A month has passed since he was detained, and his whereabouts are still unknown.
Lobsang Yeshi, a father of eight children, was a village head involved in May 2014 protests against Chinese mining in his region.
'Extremely horrible action' adds insult to injury of suspicious death of popular monk in custody, arrest of his sister.
Sonam Topgyal's will, discovered by relatives, decries repression and urges Tibetans to unite to solve the region's troubles.
Dolkar Lhamo had asked for the release of her brother's body to his family, and for an explanation of his death in prison.
Tenzin Delek Rinpoche's mouth and nails were black, leading some to fear poisoning, sources say.
The cremation takes place after Chinese authorities refuse to return the body to relatives and supporters for traditional rites.
They detained his parents and five of his siblings when they went to collect his body.
Some want his body returned home for funeral rites, while others want the remains cremated in Chengdu.
Many had traveled in secret from the religious teacher's home county in Sichuan.
Over a thousand had gathered to demand the return of Tenzin Delek Rinpoche's body to his family and hometown.
Tenzin Delek Rinpoche had been recognized by Tibet's spiritual leader the Dalai Lama as a reincarnated lama.
Security forces fill Kyegudo town and cut communications following the sixth Tibetan burning protest this year.
His burning brings to 142 the total number of self-immolations in Tibetan areas of China since 2009.