The national telecom regulator on Monday sent an urgent letter to each of the local operating units of Facebook, YouTube, Line and Twitter asking the world's popular sites to block websites insulting the monarchy.
"We need serious cooperation from Facebook and YouTube to immediately block the existing 120 webpages that we found posting inappropriate content on the famous social network and the video-sharing sites," said Takorn Tantasith, secretary-general of National Broadcasting and Telecommunications Commission (NBTC).
"We also need strong cooperation from Line and Twitter to block the usernames that we discovered were posting comments about the monarchy on the instant communication app and online social networking service," he said.
The NBTC has previously asked the social media networking giants for cooperation in the past few years, but they ignored the regulator's requests saying their web servers are not located in Thailand.
Mr Takorn said the move by the NBTC does not violate the fundamental right of people to express their point of view or state their opinions openly.
But posting comments that insult the monarchy violates Thai laws because they propagate false information with bad intention, seriously damaging the prestige of His Majesty the King.
Mr Takorn said the NBTC will send the list of 120 webpages or URLs to Facebook and YouTube service providers in Thailand by Wednesday for blocking.
People who discover inappropriate content can contact the Ministry of Digital Economy and Society's 1212 call centre, the NBTC's 1200 call centre, or email the NBTC at web_report@nbtc.go.th.
The NBTC on Monday officially opened a centre for monitoring content broadcast on radio and television as well as social media networks.
Located at the NBTC's headquarters, there are 10 staff working at the centre around-the-clock service, divided into three eight-hour shifts.