Tweets, email and instant messenger traffic went into overdrive as confused users rushed to find out what had happened to Facebook, a site used by millions of Thais but inaccessible for about 30 minutes in the afternoon.
A senior ICT ministry official confirmed the site had been blocked to thwart the spread of online criticism of the military in the wake of a May 22 coup, Reuters reported.
"We have blocked Facebook temporarily and tomorrow we will call a meeting with other social media, like Twitter and Instagram, to ask for cooperation from them," Surachai Srisaracam, permanent secretary of the Information and Communications Technology Ministry, told Reuters.
"Right now there's a campaign to ask for people to stage protests against the army so we need to ask for cooperation from social media to help us stop the spread of critical messages about the coup," he said.
Small protests have taken place daily against the regime, organised mainly on social media, testing the military as it seeks to assert its influence over the media and curtail dissent.
The junta has banned gatherings, imposed a curfew, arrested scores of activists and politicians and told print and broadcast media to refrain from critical reporting of the military. Foreign news channels like CNN, BBC and Al Jazeera have been blocked.
Small, brief protests have taken place in Bangkok as well as the northern and northeastern strongholds of the ousted government, erupting more like flash mobs than political rallies.
The military has also warned people not to spread what it considers provocative material on social media.