Troops poured into the streets and tanks surrounded the offices of Thaksin, who was out of the country at the United Nations, while military leaders said they had suspended the constitution and the government.
Thai Prime Minister Thaksin Shinawatra cancelled plans to make a speech at the UN General Assembly, following the coup attempt in Bangkok, the UN announced.
No official reason was given for the withdrawal. A UN spokeswoman, Marie Okabe, said only that the Thai mission had announced that no speech would be made.
Officials at the royal palace said top military commanders including the head of the army, General Sonthi Boonyaratglin, had met with the nation's influential King Bhumibol Adulyadej after announcing the coup. The uprising capped months of uncertainty and unhappiness about Thaksin.