The prime minister of Thailand says Muslim militants have been planning bomb attacks in Bangkok to coincide with a meeting of Asia Pacific heads of state in October.
Authorities previously insisted Thailand would not be a target
US President George W Bush is among the world leaders expected to attend.
Western embassies and tourist centres in the Thai capital were the targets of the plot,
Thaksin Shinawatra said in his weekly radio address to the nation.
He said a Singaporean man arrested last month and handed over to the authorities in Singapore had admitted to planning the attacks, although he gave no further details.
Three Thai Muslims were also arrested this week on suspicion of involvement in the plot.
A Bangkok tourist map with apparent targets circled on it - reportedly including the embassies of the United States, Britain, Israel, Australia and Singapore - was among the evidence seized from the three Thai suspects, according to the Associated Press news agency.
For months, the Thai prime minister has been insisting there was no possibility of a terrorist attack in Thailand.
Media reports of Islamic militant activity in Thailand were, he said, the work of "crazy people".
But on Saturday morning, Mr Thaksin surprised everyone by announcing the most ambitious plot to attack Western targets in Asia since the Bali bomb last year.
News of last month's arrest was suppressed until Thaksin met Bush
The meeting of heads of state from the 21 Asia Pacific Economic Co-operation (Apec) member states will run from 20-21 October. President Bush is scheduled to attend.
The suspect, a Singaporean Muslim, was arrested in Thailand last month and is since alleged to have admitted being a member of the shadowy Islamic organisation, Jemaah Islamiyah.
But the police withheld news of his arrest until this week, just as Mr Thaksin was meeting President George W Bush in Washington.
Thai police say they are now on the hunt for more members of the militant cell, including a man they say they believe is the bomb-maker.
The US has for months been pressing Thailand to take more action against militants inside its borders.
The authorities have also detained three Muslim men from southern Thailand who they say are involved in the plot.
However, they have so far produced little evidence and the arrests have been greeted with scepticism by Muslim politicians and community leaders in Thailand.
They believe the arrests are instead linked to a long-running conflict between Thailand's Muslim minority and the police.