BANGKOK (REUTERS) - Facebook blocked access within Thailand to a group with 1 million members that has criticised the country's king, but said it was planning a legal challenge to the government's demand that it block the group.

The move comes amid near daily youth-led protests against the government led by the former military junta chief and unprecedented calls for reforms of the monarchy.

The "Royalist Marketplace" group was created in April by Associate Professor Pavin Chachavalpongpun, a self-exiled academic and critic of the monarchy.

On Monday night (Aug 24), the group's page brought up a message:"Access to this group has been restricted within Thailand pursuant to a legal request from the Ministry of Digital Economy and Society."

Assoc Prof Pavin, who lives in Japan, said Facebook had bowed to the military-dominated government's pressure.

"Our group is part of a democratisation process, it is a space for freedom of expression," Assoc Prof Pavin told Reuters. "By doing this, Facebook is cooperating with the authoritarian regime to obstruct democracy and cultivating authoritarianism in Thailand."

Assoc Prof Pavin's new group of the same name already had over 455,000 members on Tuesday.

Facebook said on Tuesday it was planning to legally challenge the Thai government after being "compelled" to block access to the group.

"Requests like this are severe, contravene international human rights law, and have a chilling effect on people's ability to express themselves," a Facebook spokesperson said. "We work to protect and defend the rights of all Internet users and are preparing to legally challenge this request."

Thailand's lese majeste laws, which forbid defaming the king, with penalties of up to 15 years in prison, is often the basis for such requests to block or remove content on social media platforms.

Earlier this month, Thailand's digital minister accused Facebook of not complying with requests to restrict content, including insults to the monarchy.

On Aug 10, he gave Facebook 15 days to comply with court takedown orders or face charges under the local Computer Crime Act, which carries a fine of up to 200,000 baht (S$8,700) and an additional 5,000 baht per day until each order is observed.

Digital ministry spokesman Putchapong Nodthaisong said on Monday that Facebook cooperated before the deadline because it understood the context of Thai society.

Mr Putchapong did not comment on Facebook's plan for legal action when asked by Reuters on Tuesday.

The ministry last week filed a separate cybercrime complaint against Assoc Prof Pavin for creating the group.