Thai Prime Minister Thaksin Shinawatra has declared a state of emergency in Bangkok amid reports of a coup attempt.
Tanks took up positions outside Government House
Soldiers have entered Government House and tanks have moved into position around the building.
Mr Thaksin, who is at the UN in New York, said he had removed the chief of the army.
National television announced that forces had taken control of Bangkok "to maintain law and order", and that they were declaring loyalty to the king.
However, the BBC's Kate McGeown in Bangkok says King Bhumibol is held in high esteem by all Thais, and the declaration of loyalty does not necessarily imply that he backs the takeover attempt.
An army-owned TV station is showing images of the royal family and songs linked in the past with military coups.
The announcement said the troops belonged to the "Council of Political Reform".
BBC World, CNN and other international news channels have been taken off the air, readers in Thailand told the BBC News website by email.
Our correspondent says low-level rumours of a possible coup have been circulating for weeks.
Thai media say that two army factions appear to be heading for a clash, with one side backing the prime minister and the other side backing military commander Lieutenant General Sonthi Boonyaratglin.
The general was sacked by the prime minister earlier in the day.
Our correspondent Jonathan Head said it was not clear which faction had taken the initiative and moved into Government House.
He said there has been pressure growing on the prime minister to resign, including groups close to King Bhumibol, following a political impasse in which April's general election was declared invalid.
But it was thought that Thailand was making progress towards holding another election later in the year, our correspondent says.
Witnesses said several hundred troops were posted at key points around Bangkok, including at government installations and major intersections.
Russell Miles emailed the BBC News website to say there were troops "dressed in Swat-style gear strolling around" near Government House, and "a tense, but fairly controlled atmosphere".
He said: "We saw a group of blokes bundling a cameraman and another chap into a van. We are taking photos, but not out in the open."
At the United Nations, where the annual General Assembly is under way, it was announced that the agenda had been changed to allow Mr Thaksin to address it in the coming hours.