The military leader who led the coup has justified the takeover, saying it was directed at ending the toppled prime minister Thaksin Shinawatra's efforts to undermine the country's democratic institutions.
In the country's first coup in 15 years, General Sonthi Boonyaratklin, the army chief, led a rapid, well-orchestrated overthrow while Thaksin was away in New York.
No shots were fired during the nightime operation which started late on Tuesday.
Residents of the Thai capital awoke on Wednesday to the unfamiliar sight of soldiers on street corners and tanks blocking off the government district.
But the city of more than 10 million people was calm and most residents appeared unfazed by the dramatic turn of events.
Nearly 20 tanks, with yellow ribbons tied around their barrels, cordoned off the Royal Palace, the Royal Plaza, the army headquarters and Thaksin's office at the Government House.
The new regime has put the country under martial law and declared a provisional authority loyal to the Thai king, ordering government offices, banks, schools and the stock market to close for the day.
Television and radio stations have also been seized.
Chidchai Vanasathidya, Thaksin's deputy and most trusted aide, has been detained at the army headquarters.
Prommin Lertsuridej, the secretary to the prime minister's office, and Somchai Wongsawat, the overthrown prime minister's borther-in-law who is permanent secretary of the justice ministry, were also detained, military officials said.
Gatherings of more than five people have been banned across the country.
The coup leaders also have imposed strict controls on foreign and domestic media, state television announced on Wednesday.
Sonthi told a news conference on Wednesday that elections were unlikely before next September as it would take that long to reform Thailand's constitution but an interim prime minister will be appointed in the next two weeks.
"An interim constitution will be drafted within two weeks, and during that time a new national assembly will be appointed, and a new prime minister will be appointed".
"Nobody was behind us. We decided on our own, and we took care of it on our own ... because the people have called for it."
General Sonthi Boonyaratklin, Leader of Thailand's coup
The army general also moved to reassure world leaders that the country's foreign policy and international agreements would be unaffected by the change in government.
Sonthi indicated that Thaksin could be prosecuted if he returns. Asked whether there would be moves to confiscate Thaksin's vast assets he said that "those who have committed wrongdoings have to be prosecuted according to the law".
Sonthi denied that King Bhumibol Adulyadej was behind the coup.
"Nobody was behind us. We decided on our own, and we took care of it on our own ... because the people have called for it and also because of the mismanagement of the government."
Thaksin is travelling from New York, where he cancelled a scheduled address to the UN General Assembly on Tuesday, to London, where he owns a home.
The ousted prime minister is expected meet up with his wife and children when his flight lands.
Soldiers are guarding the main
roads in Bangkok
The Thai News Agency reported Thaksin as saying; "I was prime minister when I came, and I was jobless on the way back," as he left New York.
Thais who trickled out onto the capital's streets overnight appeared to welcome the surprise turn of events as a necessary climax to months of demands for Thaksin to resign amid allegations of corruption, electoral irregularities and a worsening Muslim insurgency.
Many people were surprised, but few in Bangkok seemed disappointed.