Thailand' military has staged a coup, seizing government offices and television stations.
A spokesman for the coup leaders said Thailand's parliament had been dissolved and the constitution suspended.
Soldiers have entered Government House and tanks have moved into position around the building.
Witnesses said several hundred troops were posted at key points around Bangkok, including at government installations.
As the tanks rolled in, Thai Prime Minister Thaksin Shinawatra - who is at the UN General Assembly in New York - declared a state of emergency.
The forces are thought to be loyal to a military commander sacked by the prime minister earlier on Tuesday.
Supporters of the coup rallied outside UN headquarters in New York with posters condemning PM Thaksin Shinawatra.