BANGKOK: As dawn broke on Tuesday thousands of soldiers armed with automatic weapons had been deployed at key sites across the sprawling capital of Thailand.

''There are some groups with bad intentions to create unrest and threatening to use weapons on the people,'' said army commander Prayuth Chan-ocha.

People on their way to work gawked at soldiers in jeeps mounted with machine-guns outside Central World, a luxury shopping complex burnt during unrest in 2010. Passersby took photographs of tense-looking soldiers.

Australians and other tourists visiting the city have been advised to ''exercise a high degree of caution'' after the Thai army imposed martial law at 3am on Tuesday morning, taking the government by surprise.

Army commanders made it clear they are in charge, declaring the media could not report anything that could cause disturbances. But they insisted they were not staging a coup as they have done 18 times previously since the 1930s. Some television stations were ordered to stop broadcasting regular programs.

Justice Minister Chaikasem Nitisiri told reporters the army had not consulted the government but he played down the move, saying the army was ''only taking over security''.

The army declared it has sweeping powers that can override any other laws and has set up what it called a ''peacekeeping command centre''. It declared the centre would replace the authority of a government unit that had been in charge of handling the crisis that has dragged on for six months, leaving 29 people dead and hundreds injured. It is not known how extensively the soldiers will use the law.

As protesters in rival camps were waking up on Tuesday, armed soldiers surrounded them, blocking exits.

The leader of the pro-government Red Shirt movement Jatuporn Prompan said his followers could accept the implementation of martial law but said they ''won't tolerate a coup or other non-constitutional means'' to grab power.

Anti-government protest leader Suthep Thausuban, wanted on treason charges, said last weekend he would this week fight a ''final battle'' to topple the government. He has made many similar declarations in the past.

Thitinan Pongsudhirak, a political scientist at Bangkok's Chulalongkorn University, said the imposition of martial law may be seen by the anti-government movement as support for their efforts to dissolve the government.

''It is crucial for the army to be even-handed with the anti-government and pro-government groups or we will be looking at an intensification of the turmoil,'' he said.

Thailand has been gripped by a sometimes violent power struggle for more than eight years. In broad terms the conflict pits two groups of Thai elites against each other, one backed by Bangkok's middle class and royalist establishment and the other backed by rural masses who support the family of former prime minister Thaksin Shinawatra, who lives in exile to avoid a jail sentence for corruption.

Yingluck Shinawatra, Mr Thaksin's younger sister, was forced to resign as prime minister earlier this month by judges of the country's Constitutional Court. Her supporters claimed the ruling was orchestrated by powerful figures in Bangkok.