The red shirts are demanding that fresh elections are held
The Thai government has announced that it will invoke the Internal Security Act from 11-23 March, in Bangkok and seven surrounding provinces.
The move comes ahead of a planned march by "red shirt" opposition protesters.
They are mainly supporters of the former prime minister, Thaksin Shinawatra, who was ousted in a military coup in 2006.
They say they plan to rally until the current coalition government calls new elections.
The ISA puts the military in charge, with powers to impose curfews, restrict numbers at gatherings and man check points if they deem such measures necessary.
Thai Prime Minister Abhisit Vejjajiva has cancelled a planned trip to Australia.
Fears and loathing
The last time the act was invoked was during the meeting of the regional grouping Asean held in Thailand last October - after an earlier attempt to host Asean in April was derailed by red shirt protests.
The police had failed to act against them as they took over the conference centre and the army asserted control to keep Mr Abhisit's government in power.
This time, the red shirt movement, led by the United Front for Democracy against Dictatorship (UDD), has promised a huge but peaceful demonstration.
Thaksin is living outside Thailand, but is still the focus of the protests
Smaller rallies, meetings and "political schools" are being held in various provinces before convoys of vehicles are expected to carry protesters to the capital by the weekend.
Thailand's Finance Minister Korn Chatikavanij has said the government would be "as patient as all governments need be".
"There is a very small minority who is trying to cause instability through, frankly speaking, potentially violent acts," he said.
The government also fully intended to "use all means within its powers, within the laws of the country, to make sure that the property and safety of its citizens are protected".
He admitted that political reconciliation in the country remained a distant dream.
"In fact, the situation in the next few years might indicate that in fact political differences have actually widened over the past year," Mr Korn said.
Last month the Supreme Court ruled that just over half of the assets belonging to Mr Thaksin or his family which were frozen since the coup, should be seized.