Thai Prime Minister Samak Sundaravej has declared a state of emergency in Bangkok.
His decree gives the army control of public order after a man died in overnight clashes between pro and anti-government protestors.
Under the sweeping emergency powers announced on television and radio, all public gatherings in the capital are banned, as well as media reports that 'undermined public security', but no curfew was imposed on the city.
Mr Samak, who vowed never to bow to a street campaign to push him from power, said the security action would be restrained and not last more than a few days.
Although the deployment of troops will come as welcome relief to the overstretched police, it raises the spectre of an army seizure of power less than two years after the military kicked out then Prime Minister Thaksin Shinawatra.
Mr Samak faced a new threat today when the Election Commission recommended his ruling People Power Party be disbanded for vote buying in last December's general election.
It could be months before the courts give their ruling, but if they agree with the commission, he and other top party leaders could be banned from politics for five years.
Army commander Anupong Paochinda, the man now in charge of the streets of Bangkok, is meeting with security officials to decide how to implement the emergency order.
At Government House, leaders of the PAD, the protest movement that has occupied Mr Samak's official compound for the past week, vowed to stay behind its barricades of razor wire and car tyres.
Some schools and shops were shut in Bangkok but traffic flowed, with no major troop presence or tanks in the streets.
The airport, the main gateway for millions of tourists visiting one of Asia's top holiday destinations, remains open.
At least one man was killed and 34 hurt in the overnight clashes between the PAD and pro-government supporters, the worst outbreak of violence since the PAD launched its street campaign.
The PAD, a group of right-wing businessmen and activists whose 2006 street campaign contributed to the coup against Mr Thaksin, says Mr Samak is an illegitimate proxy for the former telecoms billionaire, now in exile in London.
The PAD also paints itself as a guardian of revered King Bhumibol Adulyadej against a supposed campaign to turn Thailand into a republic, a charge denied by both Mr Thaksin and the government.
Meanwhile, the Department of Foreign Affairs has warned Irish citizens travelling to Bangkok and other areas of political unrest in Thailand to exercise extreme caution.
The department's website warns that violent clashes in Bangkok are becoming increasingly volatile, unpredictable and dangerous and says that people should avoid demonstrations and large gatherings and stay away from government buildings.