A Thai man has been jailed for two years for selling calendars which featured satirical comments and rubber ducks in royal regalia, which prosecutors said defamed the monarchy.

Narathorn Chotmankongsin, 26, has been convicted of insulting the Thai king.

He is among about 200 people who have been arrested under lese majeste laws since 2020 in what critics say has been a crackdown on free speech.

The rubber duck has been a symbol of pro-democracy protesters in Thailand.

Activists widely used the symbol in demonstrations where they called for a democratic transition - a movement which also includes demands for reforms to the monarchy.

Narathorn was arrested in December 2020 for selling the calendars on the pro-democracy Facebook page Ratasadon. The political satire featured illustrations of ducks in royal regalia, and controversial captions.

Prosecutors argued the images and descriptions ridiculed and defamed King Maha Vajiralongkorn.

A court on Tuesday sentenced Narathorn to three years, before commuting the sentence to two years.

Human Rights Watch (HRW) said the sentence showed Thai authorities were punishing any activity they deemed insulted the monarchy.

"This case sends a message to all Thais, and to the rest of the world, that Thailand is moving further away from - not closer to - becoming a rights-respecting democracy," said HRW Asia director Elaine Pearson.

Rights groups have accused the Thai government of exploiting the royal insult law in recent years to clamp down on political dissent.

The youthful protests which broke out in Thailand in July 2020, and continued into 2021, shocked conservative royalists. For the first time young activists demanded reforms to the country's powerful monarchy, and during their street rallies openly mocked members of the royal family.

Critics also say the scope of what counts as an offence is widened. For example, protesters wearing skimpy crop-tops, similar to those sometimes worn by the king, and a protester wearing a pink dress similar to one worn by Queen Suthida were prosecuted.

The criminalisation of cartoons featuring yellow ducks is another leap in the use of the law. The giant inflatable ducks became a feature of the 2020 protests when they were brought in to add an element of fun and ridicule in response to the heavy-handed riot-control tactics used by the police.

They were not viewed at the time as satirising the monarchy, although the prosecution alleged at the trial that the way the ducks were dressed in the defendant's calendars did suggest a link with the king.

Thailand is frequently criticised both for the very broad interpretations it allows of the lese majeste law, for the secrecy of the trials and the routine denial of bail, as well as the almost 100% conviction rate, and for the severity of the sentences.

However Prime Minister Prayuth Chan-ocha has rejected this criticism. The government still insists the law is needed to defend the monarchy, the survival of which is officially viewed as essential to Thailand's national identity.

Thai authorities have also increasingly used the country's computer crime laws to prosecute those who've posted critical comments on social media.

On Wednesday, two young activists currently detained under lese majeste charges entered their 50th day of a hunger strike protesting against the strict law.