A Thai man was jailed for 30 years on Friday for "insulting" the monarchy on Facebook, in one of the toughest known sentences passed under the junta-ruled kingdom's draconian lese majeste law.

Thai King Bhumibol Adulyadej, 87, is protected by one of the world's strictest royal defamation rules under which anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

On Friday Bangkok's Military Court found Pongsak Sriboonpeng, 48, guilty of posting messages and pictures defaming the monarchy in six posts on the social networking website.

He was sentenced to 10 years on each count with the 60-year jail term halved after he pleaded guilty, his lawyer Sasinan Thamnithinan told AFP.

"It's broken the record," she said about the severe jail term, adding that because Pongsak was arrested while Thailand was still under martial law there was no right to appeal the sentence passed by the military court.

Lese majeste convictions have surged since Thailand's generals seized power from an elected government in May 2014.

According to iLaw, a local rights group that monitors such cases, there were just two ongoing prosecutions for royal defamation before the coup. Now that number is at least 56.

Critics of the law say it has been used as a weapon against political enemies of the royalist elite and their military allies and now targets those opposed to the coup.

In another conviction this week a military court in the northern province of Chiang Rai sentenced a man with a history of mental illness to five years in jail for lese majeste.

Samak Pantay, 48, was found guilty on Thursday of slashing a portrait of the king and queen in July last year, lawyer Anon Numpa said.

"He confessed to the charge so the judge commuted the sentence to five years," he said, adding that Samak has been medically certified as mentally sick for "more than 10 years".

Thailand's ultra-royalist generals have long used their self-appointed position as defenders of the monarchy to justify coups and political interventions in the country's often turbulent politics.

But both Thai and international media must heavily self-censor when covering lese majeste and the monarchy -- even repeating details of charges of perceived defamation offences could mean breaking the law.

Thai authorities rarely provide details of cases, leaving rights groups to follow prosecutions across the country.

In April a businessman was jailed for 25 years for posting Facebook messages deemed to be defamatory to the monarchy in a ruling rights groups described as one of the harshest known.

Thailand's ruling military replaced martial law with new powers in April retaining much of the same authority but under the new orders civilians can now appeal to a higher tribunal for lese majeste crimes, though they are still tried at a military court.