The surprise announcement in Bangkok was made before dawn on Tuesday. It was not immediately clear whether a coup d'etat was under way.
Lt. Gen. Nipat Thonglek told CNN the move is not a coup and said details of the restrictions under martial law were being worked out.
The move came after six months of anti-government demonstrations aimed at ousting the government and one day after the Southeast Asian country's caretaker prime minister refused to step down.
The army said in a statement that it had taken the action to "keep peace and order" and soldiers entered several private television stations that are sympathetic to protesters.
A ticker on Chanel 5, an army station, however, denied the military was taking over, saying "the invocation of martial law is not a coup".
Thailand's army has staged at 11 successful coups since the end of absolute monarchy in 1932.
