All radio and TV channels broardcast military statement of successful coup late Tuesday and witnesses saw that forces seized the prime minister's office in Bangkok.
Earlier, Thai military TV Five reported that the Thai military declared launch of coup.
Over a dozen tanks blocked roads around Thailand's government headquarters in Bangkok and witnesses saw armed soldiers in the streets of the capital.
Thai caretaker Prim Minister Thaksin Shinawatra, who is in New York at a United Nations meeting, announced a state of emergency in Bangkok.
"I declare Bangkok under a severe state of emergency," he said in a voice broadcast on Thai television. Thaksin ordered troops not to "move illegally".
He said he was ordering the transfer of the nation's army chief to work in the prime minister's office, effectively suspending him from his military duties.
The statement was suddenly cut off after airing for about 13 minutes.
Several hundred soldiers were deployed at keys points in the capital of Bangkok.
Army-owned TV channel 5 interrupted regular broadcasts with patriotic music and showed pictures of the king.