Thailand's information technology ministry says it blocked Facebook and plans to hold talks with other social networking sites to stop protests against the military government.

"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."

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Print and broadcast media have already been instructed not to publish any reporting critical of the military's May 22 takeover.

Facebook users in Thailand reacted with alarm after experiencing widespread problems accessing the social networking site on Thursday.

"Urgent: Facebook has been suspended," one user wrote on Twitter.

"Surely that would be suicide. Whole country would protest," wrote another user.

But just minutes later users celebrated: "Facebook is back!!"

An Australia Network audience member tweeted that the site was down for 40 minutes.

Army denies block

The military regime said it had not pulled the plug on the social media site.

"We have not ordered a block of Facebook - it's not our policy," said army spokesman, Colonel Winthai Suvaree.

"Facebook experienced a slight technical failure and the Ministry of Information and Communication Technology is working to fix it now," he said.

But some users were unconvinced, speculating that it could have been a trial run for a possible blackout in the future, or a warning shot to social media users not to criticise the coup.

Social media platforms such as Twitter and Facebook are hugely popular in Thailand.

The junta had warned it would block any social media platforms found to carry content that incites violence or is critical of its military leaders.

Red Shirts freed

Meanwhile, the junta has freed leaders of the "Red Shirt" movement allied to the civilian government ousted in a recent coup, after holding them without charge for nearly a week.

Key members of the protest group walked out of an army facility in Bangkok, according to an AFP journalist at the scene.

Thida Thavornseth, a core Red Shirt leader, was among four senior members seen leaving the military building, none of whom gave any comment to the media.

She was detained last week, along with former premiers Yingluck Shinawatra and Abhisit Vejjajiva, during a meeting supposedly aimed at bringing Thailand's warring political factions together which instead ended dramatically with the army seizing power.

Jatuporn Prompan, who was shown on national television late Tuesday being held at an undisclosed army facility, was also released, according to his official Facebook page.

The army has said people who have been detained and released since the military takeover must sign a document promising to cease political activity.

AFP/ Reuters