Thai Prime Minister Yingluck Shinawatra, who issued the special internal security order for the capital and surrounding districts during a television address, said the protests, which began three weeks ago, were no longer peaceful rallies as the participants were resorting to occupying state offices, tearing down gates, and cutting off power and water supply, threatening national security and that the new order was needed to maintain law and order.
She, however, said her government would not use force against the protesters and that the ISA would follow international norms, the Bangkok Post reported.
The new security order would allow authorities to impose curfews, and set up checkpoints and restrict the movement of demonstrators.
The protesters, who are demanding the resignation of the Yingluck Shinawatra government, forced their way into the Thai Finance Ministry Monday and vowed to occupy more state offices. Some reports said the protesters broke and crossed police lines near the state offices.
The protests were being led by former Deputy Premier Suthep Thaugsuban with the opposition Democrat Party.
On Sunday, there were reportedly about 100,000 protesters in Bangkok. There were also rallies by tens of thousands of people supporting the government.
"Our protest will not stop until Thaksin's regime is wiped out," Thaugsuban said, CNN reported.
Yingluck is the sister of former Premier Thaksin Shinawatra, a wealthy telecommunications businessman who was ousted in a 2006 coup and has been living in exile since his ouster. Critics accuse Yingluck, who they see as a proxy for her brother, of trying to pave the way for Thaksin's return to Thailand.
The protests are raising concern in the United States, which considers Thailand a close ally and critical for its new Asia-Pacific pivot.
In a statement Monday, State Department's Jen Psaki said the U.S. government is "concerned about the rising political tension in Thailand" and is following the protests in Bangkok closely.
Noting violence and seizure of public or private property "are not acceptable means of resolving political differences," Psaki said the United States, as a long time friend of Thailand, firmly believes all parties should resolve differences through peaceful dialogue.
Yingluck has refused to resign or dissolve Parliament.
"I will fully explain every issue during the censure (debate in parliament) and in order to overcome time constraints, may also issue additional clarification at other venues," she said Monday, Bangkok Nation reported.
The report said junior coalition parties in the government will not desert her government.
Opposition Democrat Party leader Abhisit Vejjajeya said about 20 lawmakers were standing by to expose the government's alleged corruption scandals during Parliament debate, the Nation reported.
The current protests were triggered after Yingluck came under fire for backing an amnesty bill critics contend is designed to allow Thaksin to return to Thailand. Earlier this month, the Thai Senate rejected the bill, but that has not helped stop the protests.
From March through May of 2010, thousands of pro-Thaksin red shirt protesters occupied parts of Bangkok. After weeks of negotiations with no resolution, the Thai military was called in to end the protest, which resulted in the deaths of about 90 people.
The New York Times said Thaksin, who lives overseas after being convicted of abuse of power, still enjoys support in the northern and northeastern provinces of Thailand.
Some analysts told the Times they think his party, Pheu Thai, could win an election if held.
His opponents told the Times Thaksin has gained much power in Thailand and seems to be calling the shots even while in exile.
"The protest leaders need to clarify their demands," Yuttaporn Issarachai, political science dean at Sukhothai Thammathirat Open University, was quoted as saying on Thai television.
The Times said Yingluck, during her 30 months at the helm, has been promoting populist policies, even though some of her programs have put a strain on state coffers.