Thai Army Chief General Prayuth Chan-O-Cha also invited both the country's rival political blocs for talks, hours after imposing martial law.
"We are in the process of inviting both sides to talk but at the minute the situation is still not normal... That's why I have had to invoke martial law," Prayuth told reporters.
The move was announced on the military's television channel at 3 AM (1:30 IST).
"People are urged not to panic, and can carry on their business as usual. Declaring martial law is not a coup d'etat," it said.
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An army spokesman said that the imposition of martial law will have no impact on the caretaker government which remains in office. The announcement also granted the army wide-ranging powers to enforce its decision.
In response, acting Prime Minister Niwattumrong Boonsongpaisan urged the army to act "under the constitution" and "with no violence".
"They took this action unilaterally. The government is having a special meeting regarding this," the aide said, describing the situation as "half a coup d'etat".
The military's decision was approved by Prayuth, citing a 1914 law that allows it to intervene during times of crisis.
Prayuth called on the rival parties to talk to each other and resolve the political crisis. Martial law would remain in place until "peace and order" had been restored, he told government officials today, the BBC reported.
Martial law comes after a long-running political crisis, and months of escalating tensions between the government and the opposition. Anti-government protesters have staged mass protests in recent days to topple the government.