Thailand's powerful army has declared martial law, deploying troops into the heart of Bangkok in a dramatic move it said is aimed at stabilising the country after six months of turbulent political unrest.

The surprise operation, which the military insisted is not a coup, places the army in charge of public security nationwide, and came amid deepening uncertainty over the nation's fate and one day after the caretaker prime minister refused to step down in the face of long-running anti-government protests.

Although soldiers entered multiple television stations to broadcast the army message, life in the vast skyscraper-strewn metropolis of 10 million people remained largely unaffected, with schools, businesses and tourist sites open and traffic flowing as usual.

On a major road in front of one of the country's most luxurious shopping centres, bystanders watched as soldiers in jeeps mounted with machine guns briefly diverted traffic. The mood was not tense; passers-by stopped to take mobile phone pictures of the soldiers.

Thailand, an economic hub for south-east Asia, has been gripped by off-and-on political turmoil since 2006 when former PM Thaksin Shinawatra was toppled by a military coup after being accused of corruption, abuse of power and disrespect for King Bhumibol Adulyadej.

His overthrow triggered a power struggle that continues to this day and in broad terms pits Thaksin's supporters among a rural majority in the north and north-east against a conservative establishment in Bangkok and the south.

The army, which is seen by many as sympathetic to anti-government protesters, has staged 11 coups since the end of absolute monarchy in 1932. But it made no moves today to dissolve the country's constitution or its current caretaker government.

Acting prime minister Niwattumrong Boonsongpaisan - who was not consulted beforehand on the army move - called an emergency cabinet meeting to discuss the situation at an undisclosed location.

In a brief statement, Mr Niwattumrong said only that the government hopes the military action will "bring peace back to the people of every group and every side".

Education minister Chaturon Chaisang, however, said in a post on his Facebook page that martial law was not an answer and warned it could "eventually spiral into a situation in which the military has no choice but to stage a coup".

Thailand's problems are "fundamentally political problems that must be solved through political processes under democracy... not military or security measures", Mr Chaturon said.

The military statement was issued today by army chief Gen Prayuth Chan-Ocha, who cited a 1914 law that gives the authority to intervene during times of crisis. He said the military took action to avert street clashes between political rivals which he feared "could impact the country's security".

He said: "The Royal Thai Army intends to bring back peace and order to the beloved country of every Thai as soon as possible. We intend to see the situation resolved quickly."

Gen Prayuth later called a meeting with senior officials from government agencies, provincial governors and representatives from the country's independent agencies - but not the cabinet.

The latest round of unrest started last November, when demonstrators took to the streets to try to oust then-PM Yingluck Shinawatra, Thaksin's sister. She dissolved the lower house of parliament in December in a bid to ease the crisis, and led a weakened, caretaker government with limited powers since then.

Earlier this month, the Constitutional Court ousted Ms Yingluck and nine cabinet ministers for abuse of power. But the move, which left the ruling party in charge of government, did little to resolve the conflict.

Competing protests in Bangkok have raised concerns of more violence, which were heightened by anti-government protesters who set a Monday deadline for achieving their goals of ousting the remnants of the government.

An overnight attack last week on the main anti-government protest site left three dead and more than 20 injured. It raised the toll since November to 28 dead and drew a strong televised rebuke from the army chief.

"This week looked ominous," said Thitinan Pongsudhirak, director of the Institute of Security and International Studies at Bangkok's Chulalongkorn University. "There was a strong likelihood of violence and turmoil.

"Martial law is intended to impose peace and order, but the key will be the army treatment of the two sides," Mr Thitinan said. "If the army is seen as favouring one side over the other, then we could see the situation spiral and deteriorate. If the army is seen as even-handed... we could actually see the situation improving."