It also followed threats by anti-government protesters to intensify their campaign to oust the ruling party, and an attack last week on protesters that killed three people and injured over 20.
The military, which has staged 11 successful coups since the end of absolute monarchy in 1932, is widely seen as sympathetic to the protest movement.
Cabinet ministers said army chief Gen. Prayuth Chan-Ocha did not consult the government before issuing the surprise announcement Tuesday to take charge of security nationwide.
Although soldiers entered multiple television stations to broadcast the army message, life in this vast skyscraper-strewn metropolis of 10 million people and the rest of the country remained largely unaffected, with schools, businesses and tourist sites open and traffic flowing normally.
Near one of Bangkok’s most luxurious shopping malls, bystanders stopped to snap smiling “selfies” of themselves with armed soldiers in jeeps and Humvees.
In the military announcement, Prayuth cited a 1914 law giving authority to intervene during crises. He said the military was acting to prevent street clashes between political rivals, and that it would “bring back peace and order to the beloved country of every Thai as soon as possible.”
Speaking to reporters later, Prayuth said martial law would last until “there is stability,” and that it was needed to force the two sides to talk about a solution.
“Don’t worry,” he said. “Everything will still go on normally. (We) will try not to violate human rights — too much.”
Acting Prime Minister Niwattumrong Boonsongpaisan called an emergency Cabinet meeting at an undisclosed location. Afterward, he issued a brief statement saying only that the government hopes the military action will “bring peace back to the people of every group and every side.”
Education Minister Chaturon Chaisang, however, said in a post on his Facebook page that martial law was not an answer and warned it could “eventually spiral into a situation in which the military has no choice but to stage a coup.”