David Kaye, the UN's special rapporteur, on Tuesday attacked the case against Jatupat Boonpatararaksa - a pro-democracy student activist - who was arrested in December for sharing a profile of King Maha Vajiralongkorn on Facebook.

"Public figures, including those exercising the highest political authority, may be subject to criticism, and the fact that some forms of expression are considered to be insulting to a public figure is not sufficient to justify restrictions or penalties," Kaye said in a statement.

Kaye said such laws "have no place in a democratic country" and called for Thailand to repeal them.

The chances of that happening are low as the country is run by royalist generals who have formed the most authoritarian government for many years, with political gatherings banned and opponents jailed or facing trial.

Use of lese majeste law - making it illegal to defame, insult or threaten the monarch - has leapt since ultra-royalist generals seized power in 2014.

Jatupat Boonpatararaksa - a prominent junta critic better known by his nickname Pai - is the only person to have been prosecuted so far and faces up to 15 years in prison if convicted.

The Ministry of Foreign Affairs said it could not comment on the case against Pai as it was ongoing.

Meanwhile, a Thai court on Tuesday sentenced an Australian man to death for murdering a Hells Angels gang member in 2015, a lawyer at the trial said.

jh/rt (AFP, AP, Reuters)