"We call for the immediate release of all those who have been jailed or held in prolonged pre-trial detention for the exercise of their rights to freedom of expression," said Ravina Shamdasani, spokesperson for the Office of the High Commissioner for Human Rights (OHCHR).
"We also urge the military government to amend the vague and broad lese-majeste law to bring it in line with international human rights standards," Shamdasani said.
The UN's call came after the Bangkok military court sentenced Pongsak Sribunpeng to 30 years in prison on August 7 for posting six comments that were critical of the Thai Royal Family on Facebook.
The sentence initially of 60 years -- 10 years for each of his posts -- was reduced to 30 years on guilty plea.
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On the same day, the military court in Chang Mai handed a 28-year prison term to a hotel staff woman, Sasiwimol Patomwongfa-ngarm, for seven posts on Facebook critical of the monarchy.
Section 122 of the Thai Criminal Code, also known as lese-majeste (injured majesty) laws is the draconian royal defamation law under which a person could be jailed for 15 years on each count for criticising the king, queen, regent or crown prince.
"We are appalled by the shockingly disproportionate prison terms handed down over the past few months in lese-majeste cases in Thailand," said Shamdasani.
"These are the heaviest sentences that we have recorded since 2006 when we began documenting cases of individuals prosecuted for lese-majeste offences for exercising their right to freedom of expression. And there has been a sharp increase in the number of such cases," she said.