Two young Thais accused of defaming the monarchy in a university play have been jailed for two and a half years, as the ruling military government intensifies a crackdown under the controversial lese majeste law.

Student Patiwat Saraiyaem, 23, and activist Porntip Mankong, 26, were sentenced on Monday after pleading guilty to defamation following their arrests last August, nearly a year after "The Wolf Bride", a satire set in a fictional kingdom, was performed.

The pair were originally sentenced to five years in prison each but the term was reduced to two years and six months due to their confessions, said a judge at Ratchada Criminal Court in Bangkok.

"The court considers their role in the play caused serious damage to the monarchy and sees no reason to suspend their sentences," he said.

The pair had each been charged with one count of lese majeste linked to the performance at Bangkok's Thammasat University, which marked the 40th anniversary of a pro-democracy student protest at the campus that was brutally crushed by the military regime in October 1973.

After the sentencing, the pair's lawyer Pawinee Chumsri said the two "would not appeal" the ruling.

Police are hunting for at least six others involved in the play for allegedly violating "112" - the feared section of the Thai criminal code which carries up to 15 years in jail for each count of insulting the king, queen, heir or regent.

Of those on the wanted list, at least two have fled Thailand.

Thailand's King Bhumibol Adulyadej, 87, is revered by many in the country as a demi-god and shielded by one of the world's most draconian royal defamation laws.

Critics say the lese majeste law has been used as a tool to suppress political dissent, noting that many of those charged have been linked to the opposition Red Shirt movement.