Army chief Prayuth Chan-ocha has declared martial law because he wants to use it as a lever to get all sides in the political conflict to negotiate and break the deadlock, according to an army source.
The army chief also wants to prevent any further violence and to respond to criticism that the army has failed to stop violence during the six-month long anti-government protests, in which 28 people have been killed and more than 700 injured.
The source said Gen Prayuth decided to declare martial law without consulting other armed forces leaders or the permanent secretary for defence because intelligence reports had indicated war weapons would be used to instigate further violence.
Gen Prayuth is insistent that there must not be any further loss of life so made the decision on Sunday to draft an announcement to declare martial law, the source said.
The army chief informed his close aides and relevant military unit commanders on Monday at 7pm. Soldiers on the ground were told around midnight and were sent to several key spots including television stations at 3am yesterday when martial law took effect, the source said.
Gen Prayuth then called leaders of the other armed forces and Supreme Commander Tanasak Patimapragorn, who was on a visit to Pakistan at the time, to inform them of his decision.
The source also said Gen Prayuth had told his subordinates that the imposition of martial law was not intended to stage a coup, but to take control of security operations from the police force and Interior Ministry.
Another source at a meeting held yesterday afternoon between the army chief and government agencies, said Gen Prayuth told officials the military will oversee security operations but not get involved in whether the country needs an appointed interim prime minister or fresh elections.
The army leader meanwhile criticised Tarit Pengdith, chief of the Department of Special Investigation and secretary-general of the now-dissolved Centre for the Administration of Peace and Order, for complicating the political situation by filing criminal charges against government opponents.
Gen Prayuth said the country is in a mess and Mr Tarit should stop filing charges, the source said. "It's all a big mess and you should stop," Gen Prayuth reportedly told Mr Tarit during the meeting, adding: "If you want to sue anyone come and sue me."
Mr Tarit later told reporters that he had not felt pressured to step aside by the army, calling the military's decision to step in "a good thing".