Thai police used tear gas and water cannon to defend the besieged government headquarters as leaders of opposition protests gripping the capital gave the prime minister two days to hand "power to the people".
The ultimatum came after talks between the two sides ended in failure, with protesters vowing to keep up their campaign to overthrow Prime Minister Yingluck Shinawatra, following street violence which left four dead and dozens wounded.
The bloodshed is the latest in a series of outbreaks of civil strife to rock the kingdom since royalist generals ousted billionaire tycoon-turned-premier Thaksin Shinawatra, Yingluck's brother, seven years ago.
Police repeatedly fired tear gas and water cannon as a hard core of protesters tried for hours to breach barricades and cut barbed wire protecting Government House, which was heavily guarded by security forces including unarmed soldiers.
Protest leader Suthep Thaugsuban later revealed that he had met Yingluck in secret in the presence of the army, navy and air force commanders but without progress.
"I told Yingluck that this is the only and last time I see her until power is handed over to the people," Suthep said said in a televised speech.
"I told her the only solution is to hand over power to the people. There will be no bargaining and it must be finished in two days."
The street rallies, the biggest since mass pro-Thaksin protests in Bangkok three years ago left dozens dead in a military crackdown, are aimed at replacing Yingluck's government with an unelected "people's council".
Suthep said the demonstrators would not be satisfied with new elections, raising fears of a deepening crisis that could scare off foreign tourists and international investment in one of Southeast Asia's most vibrant economies.
An estimated 70,000 people joined the opposition demo on Sunday, according to Police Major General Anucha Romayanan.
As night fell, protesters armed with iron bars were seen near Government House.
In a televised press conference, deputy prime minister Pracha Promnog advised the public to stay at home between 10 pm and 5 am for their safety.
"The government is in control and will restore normality as soon as possible," he added.
He accused protest leader Suthep Thaugsuban of seeking "to overthrow the executive branch, which is treason and punishable by death".
Violence broke out late Saturday in the area around a suburban stadium where tens of thousands of pro-government "Red Shirts" had gathered in support of Yingluck, who has faced weeks of street protests.
Four people were killed and 57 wounded, according to Bangkok's Erawan emergency centre. The dead and injured suffered a range of wounds including gunshots and stabbings.
At least two of the dead were believed to be Red Shirt supporters.
The circumstances were unclear but the violence came after an anti-government mob attacked Red Shirts arriving to join the rally in Ramkhamhaeng district.
They were the first deaths since the mostly peaceful demonstrations began a month ago. Both sides blamed each other for attacking their supporters.
Protesters intensify their campaign
The violence prompted Red Shirt leaders to end their rally, which had drawn tens of thousands of mainly rural poor in support of Yingluck and her brother Thaksin, who lives in self-imposed exile but remains a hugely divisive figure in Thailand.
Protest leader Suthep urged all civil servants to go on strike on Monday although it was unclear how many would heed his call, which was rejected by the government.
While the protesters' numbers have fallen sharply since an estimated 180,000 people joined an opposition rally on November 24, they have increasingly sought out high-profile targets in what experts believe could be an attempt to provoke a military coup.
The kingdom has seen 18 actual or attempted coups since 1932, most recently with Thaksin's overthrow in 2006, but the military has appeared reluctant to intervene in the current standoff.
Authorities are deploying more than 2,700 troops to reinforce security in Bangkok, the first time a significant number of soldiers has been mobilised to cope with the unrest.
The protests were triggered by an amnesty bill, since abandoned by the ruling party, which opponents feared would have allowed the return of Thaksin, whose overthrow by royalist generals in 2006 unleashed years of political turmoil.
Thaksin is adored by many of the country's rural and urban working class for his populist policies while in power, but hated by many southerners, middle-class Thais and the Bangkok elite, who see him as corrupt and a threat to the monarchy.
Pro-Thaksin parties have won every election for more than a decade but Yingluck has given no indication that she is thinking of calling fresh polls as a way out of the crisis.