Military and police seized control of Thai capital Bangkok on Tuesday night amid reports of an apparent military coup against Prime Minister Thaksin Shinawatra, local television said.
An announcement flashed on national television said forces, loyal to the king, had taken control of Bangkok "to maintain law and order".
The announcement said the troops belonged to the 'Council of Political Reform'.
However, it was unclear if they sided with Thaksin or military commander Lieutenant General Sonthi Boonyaratglin, who was sacked earlier by the premier.
Tanks and armed soldiers were seen in the streets of Bangkok.
Shinawatra has declared a state of emergency, reports said.