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 law - which carries a maximum 15 year jail sentence - has a broad definition as to what constitutes an insult to the king, his successor or regent, and the queen.

However Prime Minister Prayuth Chan-ocha has rejected this criticism. His government argues the law is necessary to protect the monarchy, which is widely revered in Thailand.

In recent times, people have been prosecuted for dressing like the king's consort or insulting the king's dog.

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.