Hundreds of Thai troops armed with assault rifles are blocking Bangkok's main business district from thousands of anti-government protesters.
The government earlier declared Silom Road, a thoroughfare studded with banks and office buildings, off limits to the protesters who have camped in the capital's main shopping district nearby for weeks.
Soldiers also patrolled the city's most famous bar strip, Patpong Road, and an entertainment area for Japanese tourists, which are just off Silom Road.
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Some took positions atop buildings after searching for possible snipers.
As Thailand's traditional New Year holiday ended, protest leaders had called for a mass rally at Silom on Tuesday in their bid to oust Prime Minister Abhisit Vejjajiva.
That has raised concerns of more clashes after savage fighting a week ago killed 25 people.
Main shopping area 'unsafe'
The military declared the city's main shopping boulevard unsafe on Sunday because of large crowds of anti-government protesters, and soldiers were sent to nearby high-rises to watch for any violence.
The warning by army spokesman Colonel Sansern Kaewkamnerd was another blow for Thailand's vital tourism sector, which accounts for six per cent of the economy and has steeply declined since the protests began on March 12.
Sansern said military checkpoints were being set up at entry points to the capital and within Bangkok to try to prevent more "Red Shirt" protesters from reaching the main rally site.
The protesters consist mainly of poor rural supporters of former Prime Minister Thaksin Shinawatra and pro-democracy activists who opposed the military coup that ousted him in 2006.
Meanwhile, the rival, establishment-backed "Yellow Shirt" protest movement vowed on Sunday to take action unless the government "strictly and efficiently enforces the law" to deal with the crisis.
The yellow-shirted protesters led months of anti-Thaksin rallies that led to the coup and their 2008 demonstrations shut Bangkok's airports for a week to protest a Thaksin-allied government.
They retreated after Abhisit's arrival but many fear their return if he is forced out.
