At least one person died and more than 30 were injured in the overnight clashes in the streets between supporters of the pro-government National United Front of Democracy against Dictatorship (DAAD) and the anti-government People's Alliance for Democracy (PAD), the Thai News Agency reported.
The emergency order banned gatherings of more than five people, the report said. Local media would be prevented from disseminating any news that may be seen as causing public panic or instigating violence.
The emergency decree would be enforced by a team to be headed by Thai Army Chief Gen. Anupong Paochinda. The team includes the national police chief and the first regional army commander.
Despite the decree, the PAD vowed to continue its protest on the Government House compound until the prime minister resigned, the report said.
Schools and colleges in the areas where the clashes occurred have closed indefinitely.
The Samak government has been under public pressure for weeks. PAD supporters accuse the government of being a proxy for former Prime Minister and billionaire Thaksin Shinawatra, who was ousted in a coup in 2006.