Thai Prime Minister Abhisit Vejjajiva declared a state of emergency in Bangkok and surrounding provinces on Sunday in a bid to quell anti-government protests that forced the cancellation of a major Asian summit the previous day.
Abhisit, in a TV address, said the nation was "in danger" because of unreasonable, self-serving people. The prime minister, who had narrowly escaped being beaten up himself by protesters, warned that a line had been crossed and a very firm response was required.
"The next three or four days will be crucial to returning to peace," he added.
Troops were seen securing strategic road junctions in the city and the capital's glitzy big department stores closed early.
A protest leader, Jatuporn Promphan, a leader of the so-called Red Shirt protesters loyal for fugitive former premier Thaksin Shinwatra, had called for a popular uprising on Sunday to overthrow what he described as an illegitimate government.
Thaksin himself, in a telephone address from an undisclosed foreign country, again incited a large body of protesters in the early evening in central Bangkok saying this was a "golden opportunity for the people to grab real democracy".
He also said he was ready to return to lead the country.
The leaders of the rival royalist People's Alliance for Democracy (PAD) called on Sunday for the sacking of deputy prime minister Suthep Thaugsuban, responsible for security, as well as the defence minister and the three armed forces chiefs.
The PAD, so so-called Yellow Shirts, undermined a pro-Thaksin government last year, partly by occupying Bangkok's two airports for a week in September. Chamlong Srimuang, a PAD leader and veteran activist, told reporters his movement did not want to be "suckered" into a clash with the reds, but nevertheless was ready to act unless the government finally manages to restore order.
Earlier about 50 Red Shirts forced their way into the interior ministry in Bangkok to confront the Abhisit.
A mob attacked the prime minister's car as it was leaving the ministry, smashing it with poles and plant pots. At one point a security guard fired into the air. Officials later said the Abhisit was not in his regular vehicle, but had switched to another one to exit the building.
Protest leaders also claimed to have captured at least two members of the security forces and to have detained one of the prime minister's aides.
One of Abhisit's security guards was paraded on stage at the main protest site in front of government house, injured and in handcuffs, reports from scene said.
Protesters also briefly clambered over two armoured personal carriers driving around near the upmarket Siam Paragon shopping mall in central Bangkok, according to witnesses.
There were also reports of small protests by Red Shirts in northern provinces.
An army spokesman Sansern Kaewkamnerd asked the public not to panic when it saw many armoured vehicles and armed soldiers seizing strategic intersections throughout the city.
"This is not a coup," said Sansern. A state of emergency bans meetings of more than five people and permits the government to use troops.
Armed soldiers and police patrolled the area around the royal place, parliament and government house.
Authorities arrested protest organiser Arisamun Pongruengrong, who led a mob to disrupt the Association of South East Asian Nations (ASEAN) and its dialogue partners in the seaside resort of Pattaya, 100 km south of Bangkok, Saturday.
The arrest provoked fury from Arisamun's followers, who began to converge on key government buildings and block traffic in the capital city.
Early on Sunday, it had appeared there could be a break in the protests for the important Buddhist New Year holiday. There were only about 2,000 protesters camped outside government house, police said.
Abhisit and his Democrat Party government were widely criticized in the local media on Sunday for the failure to stop the protesters from disrupting the ASEAN summit.
Many ordinary Thais remain shocked by the inability of the security forces to restrain the protesters.
"It is absolutely disgusting. This is getting completely out of hand. What the bloody hell is going on!" exclaimed Wallop Pontin, a doctor watching the protesters at a Skytrain stop in central Bangkok.
The protesters are demanding the resignation of Abhisit, whom they say was named to head the government after anti-democratic meddling from elite royalists and other influential opponents of Thaksin, who was ousted by a coup in 2006.
The flamboyant Thaksin fled Thailand to avoid a two-year prison sentence for abuse of power. But he has been agitating to return, and regularly makes live video broadcasts from abroad via satellite to rally his supporters.