Facebook blocked access within Thailand to a group with 1 million members that has criticized 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 Pavin Chachavalpongpun, a self-exiled academic and critic of the monarchy.