The UN human rights office has voiced alarm at recent arrests and jail sentences for insulting Thailand's monarchy, warning of "chilling effects" on freedom of expression under the junta.
Since the army seized power in May, at least 13 new royal defamation cases had been opened and other allegations revived, spokeswoman Ravina Shamdasani said.
"We are seriously concerned about the prosecution and harsh sentencing of individuals in Thailand under the country's lese majeste law," she said on Tuesday. "Such measures are adding to the larger pattern of increasing restrictions on freedom of expression in Thailand."
Last week, two activists were charged with breaching the strict royal insult laws during a university play in October 2013 to commemorate the 40th anniversary of a student-led uprising.
The UN said the play depicted a fictional monarch who was manipulated by his adviser.
King Bhumibol Adulyadej, 86, is revered by many Thais and protected by tough defamation laws that carry a jail term of up to 15 years for each conviction.
A 28-year-old musician was recently sentenced to 15 years' jail after he was found guilty of posting insulting messages about the monarchy on Facebook.
"The threat of the use of the lese majeste laws adds to the chilling effects on freedom of expression observed in Thailand after the coup, and risks curbing critical debate on issues of public interest," Shamdasani said.
Critics say the royal slur law has been politicised, noting that many of those charged in recent years were linked to the "Red Shirts" protest movement, which is supportive of fugitive former premier Thaksin Shinawatra.
Thaksin clashed with the royalist establishment before being overthrown in a coup in 2006.
New York-based Human Rights Watch said the arrests of the two activists, 10 months after the play was staged, suggested that the Thai junta was sending a "political message".
"A broad-based discussion is urgently needed to amend the laws to ensure that they conform with Thailand's international human rights obligations," HRW Asia director Brad Adams said.