One person was killed when a pro-government group clashed with members of an anti-government campaign that has forcibly occupied the premises of the Prime Minister's office for the past week. The emergency order puts the Army chief in charge of enforcing law and order in the capital and bans assembly of more than five people.
Restrictions have also been placed on media reports detrimental to national security.
Government schools in Bangkok have been closed for three days.
Thousands of people rallying under the banner of the civil society movement People's Alliance for Democracy (PAD) are camping inside the Prime Minister's office defying court order and police attempts to vacate.
The PAD has mobilised mass protests against former prime minister Thaksin Shinawatra two years ago leading to his ouster by a section of the armed forces.
The PAD accuses Mr Samak's government of being a proxy for Mr Thaksin after the latter was barred from public life for five years by a judicial tribunal set up following the September 2006 coup.