Thai military leaders have begun consolidating their hold on power after staging a coup while Prime Minister Thaksin Shinawatra was abroad.
Martial law has been declared, with large gatherings of people and critical news reporting banned.
Mr Thaksin's deputy and chief aide, Chidchai Vanasathidya, has been taken into army custody.
Army chief Gen Sonthi Boonyaratglin said in a TV address that the coup was necessary to unite the country.
The military had no intention of holding onto power, he added.
The coup leaders have announced that regional army commanders will take charge of areas outside the capital, Bangkok.
The country's stock market, banks and schools will be closed on Wednesday, which has been declared a public holiday.
The coup follows months of growing tension in Thailand, with protests against Mr Thaksin and a general election which was annulled due to concerns about its legitimacy.
In the capital Bangkok, tanks have cordoned off the government district. They have yellow ribbons tied around their barrels to signify loyalty to Thailand's revered king.
On the surface, Bangkok is functioning as normal, says the BBC's Kate McGeown there. The city's monorail system is packed with commuters, and cafes and street vendors are open for business.
But government offices, banks and schools are closed, and there is a strange quietness about the usually vibrant city. People are noticeably subdued, and the usual friendly chatter has been replaced by silence.
BBC World, CNN and other international TV news channels have been taken off the air, while Thai stations have been broadcasting footage of the royal family and patriotic songs.
Gen Sonthi made his brief TV address flanked by the heads of the three services and the police, and backed by giant portraits of the king and queen.
He said coup leaders had "agreed that the caretaker prime minister [Thaksin] has caused an unprecedented rift in society, widespread corruption, [and] nepotism".
He said the constitution, the senate, the house of representatives, the cabinet and the constitutional court had all been "terminated", and a newly formed Council of Administrative Reform would "restore normality".
'Not coming back'
Mr Thaksin cancelled a speech he was due to give at the UN General Assembly in New York on Tuesday evening as news of the coup emerged.
His official spokesman, Surapong Suebwonglee, initially declared confidence the coup would fail.
But contacted by Associated Press news agency, he said: "We have to accept what happened. We are not coming back soon."
The rebels have visited King Bhumibol Adulyadej and declared loyalty to him. The king has not yet made any comment on whether he backs the coup.
The coup leaders have ordered provincial governors and heads of government agencies to report to them.
They have also banned assemblies of more than five people, which are now punishable by six months in jail.
A statement urged "farmers and workers" to remain calm - an apparent reference to Mr Thaksin's rural base of support.
In another move to defend against a counter-coup, a senior general said Mr Thaksin's main deputy, Mr Chidchai, had been "invited to stay" at Army headquarters on Tuesday night.
Reaction from abroad has been largely negative.
The US called on Thais "to resolve their political differences in a peaceful manner", while the EU's Finnish presidency expressed "grave concern" at events.
Australia "deeply regretted" the coup, while New Zealand urged politicians and the military to resolve their differences democratically. Malaysia's prime minister declared he was "shocked".
UN Secretary General Kofi Annan said the organisation supported changes of government through democratic means rather than by the barrel of a gun.
The BBC's Jonathan Head in Bangkok reports that while many people wanted Mr Thaksin out of office, there will be unease about the way this has happened, and people will be looking to see whether the king has supported the coup.
Our correspondent says the coup is a surprising development for a country that had been viewed as one of Asia's leading democracies, and events in Thailand will have repercussions throughout the region.
It is the first coup attempt in 15 years in a country where they used to be commonplace. There were 17 of them between 1932 and 1991.