The Thai prime minister has invoked special powers in Bangkok and elsewhere after anti-government protesters forced their way into two key ministries.
Yingluck Shinawatra was responding to a second day of demonstrations in Bangkok by tens of thousands of people calling for her government to resign.
Her decision to enforce the Internal Security Act enables officials to impose curfews and seal roads.
The protests have been triggered by a controversial political amnesty bill.
The demonstrators, who had staged a huge rally on Sunday, marched on Monday to several different locations in Bangkok, including the police headquarters and TV stations.
The BBC's Jonathan Head in Bangkok says the first government department to fall was the finance ministry which, according to the protesters, misuses the country's budget.
The protesters then forced their way into the foreign ministry as well as the government's public relations department.
The prime minister went on national TV saying that their "illegal actions" threatened "the stability of the government" and prevented civil servants from doing their work.
However she insisted the government did not want to see a repeat of the violence that led to dozens of people being killed in a military crackdown.
"Our National Security service is now monitoring the protest and we are trying to handle the protesters without any violence," she said.
Special security measures would now be enforced in large parts of the capital, Nonthaburi to the north and areas to the east that included the airport, she added.
Campaign leader Suthep Thaugsuban - a former opposition Democratic Party lawmaker - described the protesters' entry into government buildings as a "peaceful seizure by the people" so that the "Thaksin system can no longer work".
The demonstrators say the amnesty legislation would have allowed ousted leader Thaksin Shinawatra - Ms Yingluck's brother - to return to Thailand without serving a jail sentence for corruption.
Thailand has been bitterly divided since Mr Thaksin was ousted in a military coup in 2006.
The amnesty bill failed to pass in the Senate earlier this month. But the proposal re-ignited simmering political divisions and raised the spectre of renewed political turmoil in the South East Asian nation.
'Calling the shots'
Mr Suthep had said Monday's protest would be peaceful, with crowds "blowing whistles and handing out flowers".
The demonstrators marched to state offices, military headquarters and television stations.
But at the finance ministry, hundreds of people swarmed into the compound. The police were conspicuous by their absence, our correspondent says. During the evening, they also moved into the grounds of the foreign ministry.
"Tomorrow [Tuesday] we will seize all ministries to show to the Thaksin system that they have no legitimacy to run the country," AFP news agency quoted the protest leader as saying.
Sunday's demonstration had drawn an estimated 100,000 people, who called on the government to step down.
"We have stood by silently while her [PM Yingluck Shinawatra's] brother calls the shots and she runs the country into the ground with loss-making policies," Reuters news agency quoted protester Suwang Ruangchai, 54, as saying.
About 40,000 government supporters held a separate rally in another part of the capital on Sunday.
Two years after Mr Thaksin was ousted in 2006, groups opposed to him occupied Bangkok's main airport, shutting it down.
Then in 2010, those who backed him and his allies held two months of street protests that paralysed Bangkok.
Those demonstrations ended in a military crackdown. More than 90 people - mostly civilian protesters - died over the course of the two-month sit-in.
A government led by Mr Thaksin's sister was subsequently elected and since then Thailand has remained relatively politically stable.
But the opposition accuse Mr Thaksin of running the government from self-imposed exile overseas, and the now-shelved amnesty bill has served as a spark for renewed protests.
The bill applied to offences committed during the upheaval after Mr Thaksin was removed from office. Ms Yingluck's government had argued that the legislation was a necessary step towards reconciliation.
But critics said it would allow human rights abuses - such as the killing of civilian protesters - to go unpunished.
And the opposition viewed it as a way of overturning the jail sentence given to Mr Thaksin, paving the way for his return.
Thaksin Shinawatra is a deeply polarising figure in Thai politics.
He drew huge support from Thailand's rural poor but strong opposition from other sectors in society, and the divisions dating from the 2006 coup continue to dominate the political landscape.