A Thai man was given a 50-year prison sentence Thursday for criticising the monarchy -- the longest jail term ever handed down under the kingdom's strict royal insult laws, a legal rights group said.

The record-breaking sentence comes after several years in which Thailand has ramped up use of the controversial legislation against pro-democracy protesters in what critics say is a tactic to silence dissent.

An appeal court in the northern city of Chiang Rai sentenced Mongkol Thirakot, 30, a former pro-democracy activist to a total of 50 years over posts on his personal Facebook account.

He was initially sentenced to 28 years by a lower criminal court, but was found guilty on 11 more counts during his appeal, leading to the longer sentence.

"The appeal court sentenced Mongkol Thirakot to 22 years for 112 over his 27 Facebook posts, in addition to the 28-year sentence already passed by the preliminary court. His total jail sentence is 50 years," Thai Lawyers for Human Rights (TLHR) said in a statement.

The lese-majeste law, which shields King Maha Vajiralongkorn and his close family from criticism, is often referred to as 112 in Thailand after the relevant section of the criminal code. 

TLHR said the sentence was the longest handed down for royal defamation, beating the previous record of 43 years imposed on a woman in 2021.

Mongkol, who owns an online clothing store, was first arrested in 2021 during a protest demanding the release of political prisoners.

Youth-led demonstrations in 2020 and 2021 saw tens of thousands take to the streets, with many demanding changes to the strict lese-majeste laws.

TLHR said Mongkol would apply for an appeal against the sentence at the Supreme Court.

More than 250 activists have been charged under lese-majeste laws since the 2020 protest movement began, according to TLHR.

On Wednesday, one of the main protest leaders, lawyer Arnon Nampa, had his four-year lese-majeste sentence increased by another four years.