The Thai army has imposed martial law amid a political crisis “to preserve law and order”.
The surprise announcement also granted the army sweeping powers to enforce its decision.
The military insisted it was not a coup. Soldiers have taken over TV and radio stations, and blocked off roads in the capital, Bangkok.
Martial law comes after months of escalating tensions between the government and the opposition.
Correspondents say the move could enrage supporters of the government, especially if it is seen as amounting to a coup. The army has staged at least 11 coups since the end of the absolute monarchy in 1932.
‘No need to panic’
Soldiers have moved into the main government building in Bangkok, which has been unoccupied following months of violent demonstrations by opponents who want to be rid of an administration they say is corrupt.
The military has also ordered media censorship in the interests of “national security”.
Both pro and anti-government protesters have been told not to march anywhere in order to prevent clashes.