A crowd massed before the coffin of Ampon 'Akong' Tangnoppakul outside the Bangkok Criminal Court in the north of the city where he was convicted in November for sending text messages deemed insulting to the royal family.
The 62-year-old, who was considered a "prisoner of conscience" by Amnesty International, was found dead in his cell on Tuesday. His supporters paraded his coffin through the city Wednesday to the Criminal Court building.
"I will bring his body to perform religious rites in front of the Criminal Court to remind the public of the importance of amending article 112 and allowing all those who are sick to receive medical treatment," his wife Rosmalin Tangnoppakul told reporters after collecting his coffin from hospital.
Under article 112 of the Thai criminal code, anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.
Ampon, who became known in Thailand as "Uncle SMS", pleaded not guilty during his trial, one in a series under the royal defamation legislation, which critics say is used to stifle free speech.
His conviction triggered rare public protests against the lese majeste law in the capital Bangkok in December.
"His death tells us to keep on fighting and article 112 must be changed in the future," Thai political scientist Phuangtong Pakawan told Wednesday's rally.
Many who attended were supporters of the "Red Shirts" protest movement which is loyal to ousted former premier Thaksin Shinawatra and whose members are arch-rivals of the royalist "Yellow Shirts".
"I'm not happy that Red Shirts have become involved in Akong's case," said Abhinya Sawatvarakorn, a student who has been accused of making anti-monarchy comments on Facebook.
"The problem is the justice system and the law. I don't want to see it being used as a political issue," she told AFP at the rally.
Jongjet Aoajenpong, the director of Police General Hospital, said the initial autopsy result suggested that liver cancer which spread to other organs was the cause of Ampon's death.
He said a tissue sample would be sent to a laboratory for confirmation.
The royal family is a very sensitive subject in the politically turbulent kingdom. King Bhumibol Adulyadej, 84, has reigned for 66 years but has been in hospital since September 2009.