What could happen now?
Many Thailand watchers fear the army move could enrage government supporters.
The BBC's Jonah Fisher in Bangkok says those people who voted for what is still the elected government will feel extremely frustrated by what has happened.
Most people are expecting the "red shirts", the broad protest movements linked to the government, to rally and are extremely concerned about the possibility of confrontation, he adds.
There are also worries about the economy - Thailand's currency the baht sank after the coup announcement. The stock exchange says it expects to keep operating as normal.
The government has been trying to organise a date for a new general election - those moves have been thrown into great uncertainty.