A faction of the Thai military led by the army chief says it has overthrown Prime Minister Thaksin Shinawatra.
Soldiers have entered the prime minister's offices in Government House and tanks have surrounded the building.
Mr Thaksin, who is at the UN in New York, has declared a state of emergency and said he had removed the army chief.
A government spokesman insisted the coup "could not succeed", and told the Reuters news agency that the government was still in control.
The spokesman said it had not been decided when the prime minister would return home from the UN.
However, in a television broadcast the leadership of the armed forces said it had taken control of Bangkok, declared a nationwide martial law and ordered all troops to return to their bases.
The so-called "Council of Political Reform" they announced is apparently loyal to sacked military commander Lt Gen Sonthi Boonyaratglin and has declared its 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.
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.
Political impasse
There has been pressure growing on the prime minister to resign, including from groups close to King Bhumibol, following a political impasse in which April's general election was declared invalid, says the BBC's correspondent Jonathan Head in Bangkok.
But it had been 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.
http://news.bbc.co.uk/2/hi/asia-pacific/5361512.stm
Sad, really, though very common or not so out of character really. Historically since the constitutional monarchy was established in 1932, there have been 17 or 18 coups, as I recall, some very bloody, most not.
Saw it in 1973, or was it 72 and again in 1991, missed this one, as I sold out some 5 months ago all our business investments in Thailand taking all monies out except for our two homes.
More to a Rob kop survival instincts than anything else I suppose. Like feeling an ambush, yet not know why, it just felt “like Shiite was going to happen”. It appears I may have been, in the short run, correct, yet, hope and pray this one is not as bloody as 73!!
Anyhow, in Rob Kop, some time ago got all my family to get visas just in case, have a pre-planned exit plan for all. Would advice all, who have an human interest in Thailand, better to be prepared and wrong than caught up in something one is unprepared for and wrong.
Then again thinking in terms of survival and preemptive preparedness and survival has stood me well in regards to all conduct in my personal and business affairs these many years of investing in foriegn countries.
Good luck to all, God’s grace and fate to any all and in Thailand, if I can help should it get bad, I will endeavor to do whatever I can.
TUM DII DAI DII, TUM CHUA DAI CHUA!
T HA T
IS
A LL!
