The Doha Centre has given financial aid to the family of Suwicha Thakor, who was sentenced to 10 years in prison for insulting the monarchy in postings online.
"I have to stay in jail for 10 years but five people will die", he said when he heard the verdict, referring to his dependent father, wife and three children. "I need help."
The Centre said: "This sentence is a violation of freedom of expression as guaranteed by Article 19 of the International Covenant on Civil and Political Rights, which Thailand signed in October 1996.
"We call for Thakor's immediate release, again ask Thailand to drop all proceedings for insulting the monarchy and demand the repeal of the law on lèse-majesté, which is from another age and allows random sentencing. The king is revered by most Thais, but that should not be allowed to affect freedom and democracy."
Thakor has been in Klong Prem prison, Bangkok, since his arrest on 14 January. Police claimed the IP address on his computer was used to post libellous material about the king and his family. Thakor's lawyer said the court in Ratchada, north-east of Bangkok, convicted him on 3 April of libel under the Computer Crime Act and two counts of lèse-majesté.
Prime minister Abhisit Vejjajiva's government, in power since December, is facing a groundswell of protest and has stepped up repression of Internet users in recent months. The lèse-majesté law and the 2007 Computer Crime Act force Internet Service Providers to keep user data for three months.
Any attack on the dignity of King Bhumibol Adulyadej, 81, or members of his family is punishable by a prison sentence of up to 15 years under Article 112 of the penal code.
"It's hard to know exactly how many Thais are currently accused of insulting the monarchy", Thongbai Thongpao, a lawyer specialising in human rights, told the Centre. "There simply aren't any official figures available."
On 6 March police searched the offices of the Prachatai website. Manager, Chiranuch Premchaiporn, was accused of putting out news that threatened national security and insulted the monarchy. She was freed on bail and is now awaiting trial.
In February Giles Ji Ungpakorn, a university lecturer in political science in Bangkok, left the country after proceedings were brought against him. Now living in Oxford, England, he told the Doha Centre he was sure he would not get a fair trial in Thailand. He is accused of insulting the royal family in a book about the 2006 military coup. He had also signed an open letter for the Centre headed "Why the crime of lèse-majesté is an abomination".
Briton Jonathan Head, the BBC's Bangkok correspondant, has been charged with three counts of insulting the monarchy. One is because an article on the BBC website, written by another journalist and featuring a picture of the king, was not put at the top of the page, as the Thai authorities demand.
The Doha Centre, headed by Robert Ménard, said at least nine other people were facing charges in Thailand for insulting the monarchy: Chotisak Onsoong, Darunee Charnchoensilpakul, Jakrapob Penkair, Jitra Kotchadej, Sondhi Limthongkul, Suchart Nakbangsai, Sulak Sivaraksa, Surachai Sae Dan and Thanapol Eawsakul
"The law is so vaguely worded that even praising the monarchy may in some circumstances be regarded as sarcasm and punishable by imprisonment" Thongbai Thongpao told the Doha Centre. "But reforming the law to make it more specific would also raise serious problems."
Yet in March the Thai premier told the Centre that the law would soon be "revised to clarify how it should be applied to calm fears about freedom of expression. When I took over as head of the government I met the police chief and we agreed that the law was being interpreted too freely.
"We made sure that it will be more strictly observed in future, but I can assure you that many of the cases pending have been closed. Those coming to court involve people who have put the country's institutions in danger."
More than 50 leading Thai and foreign academics, including US linguist Noam Chomsky, have written an open letter to the prime minister calling for the lèse-majesté law to be abolished.