Gun battles have erupted between Thai police and anti-government protesters in Bangkok and four people were killed and dozens wounded as authorities made their most determined effort yet to clear demonstrators from the streets.
In a day of tangled developments in Thailand's long-running political crisis, the country's anti-corruption body announced it was filing charges against Prime Minister Yingluck Shinawatra relating to a rice subsidy scheme that has fuelled middle-class opposition to her government.
The troubled rice scheme, already near collapse, suffered another blow when the Government Savings Bank said it was scrapping a loan to a state farm bank that could have been used to prop the scheme up in the face of a revolt by depositors.
The clashes were some of most intense between protesters and security forces since the campaign to unseat Yingluck began in November. The military, which has said it would intervene if police are unable to control security in the capital, has not publicly commented on the violence.
The protests are the latest installment of an eight-year political battle broadly pitting the Bangkok middle class and royalist establishment against the poorer, mostly rural supporters of Yingluck and her billionaire brother, former premier Thaksin Shinawatra.
Reuters witnesses heard gunfire and saw police firing weapons in the Phan Fa Bridge area in the old quarter of the city. Police said they had come under fire from a sniper on a rooftop and M-79 grenades were also fired.
A policeman was killed by a gunshot and several were wounded by a grenade, security officials said.
The Erawan Medical Center, which monitors hospitals, said on its website that three protesters had also been killed by gunfire. The centre said 64 people had been wounded but did not say how many were police and how many were civilians.
Security officials said earlier that 15,000 officers were involved in the operation, "Peace for Bangkok Mission", to reclaim protest sites around central Bangkok's Government House and other government offices in the north of the capital.
Yingluck has been forced to abandon her offices in Government House by the protesters, led by a former deputy premier, Suthep Thaugsuban, who have also blocked major intersections since mid-January.
Suthep told supporters at an evening rally in Bangkok's central business district that protesters would gather on Wednesday outside Yingluck's temporary offices at a Defence Ministry facility in north Bangkok.
"We are not afraid anymore. Tomorrow we will go to the Defence Ministry office... we will chase them (Yingluck and her ministers) out. No matter where Yingluck is, we will follow."
Police said they had arrested 183 people at two protest sites at the Energy Ministry, which had been cleared of protesters, and Phan Fa Bridge, and were detaining them for violating a state of emergency declared last month.
Trouble started with clouds of teargas near Government House and soon police were crouching behind riot shields as officers clashed with protesters. It was not clear who had fired the teargas and the authorities blamed protesters.
By the afternoon, police had largely withdrawn from protest sites and the streets were quiet. National police chief Adul Saengsingkaew told Reuters there were no plans to continue the operation on Wednesday.
There has been no move against the biggest protest sites in the city's commercial and shopping districts.
The protesters are trying to oust Yingluck, whom they view as a proxy for her elder brother Thaksin, a former telecoms tycoon-turned-premier, toppled by the army in a 2006 coup.
The military has remained aloof from the latest crisis, but has a long history of intervening in politics, generally in support of the Bangkok establishment that includes the top brass, royal advisers and old-money families.
"If enough people are killed then the arch-royalist grouping might pressure Yingluck to retreat from her pro-active approach towards Suthep," said Paul Chambers, director of research at the Institute for South East Asian Studies in Chiang Mai.
"I don't think the military would carry out a coup, but they would ask Yingluck to back off from Suthep."
Among the protesters' grievances is the rice subsidy scheme, a populist move to pay farmers an above-market price that has proved hugely expensive and run into funding problems.
The National Anti-Corruption Commission announced an investigation last month and on Tuesday said it was summoning Yingluck to hear charges against her on February 27.
"Although she knew that many people had warned about corruption in the scheme, she still continued with it. That shows her intention to cause losses to the government so we have unanimously agreed to charge her," Vicha Mahakhun, a member of the commission, said in a statement.
The GSB had said on Sunday it had lent 5 billion baht (NZ$185 million) to the Bank for Agriculture and Agricultural Cooperatives, which manages the rice program and has all but run out of money to pay farmers.
Some GSB depositors, either worried that the loan could destabilize the bank or unwilling to see their money used to help the government, have been taking out their cash. On Monday, 30 billion baht (NZ$1.1 billion) was withdrawn.
In a response to the withdrawals, Thaksin's son, Panthongtae Shinawatra, posted a picture on Facebook showing he had deposited just over 11 million baht in a GSB account.
The protests have taken a toll on the economy and data published on Monday showed growth slowed sharply in the fourth quarter of 2013. The baht currency weakened after Tuesday's violence.
Yingluck called a snap election in December and has since led a caretaker administration with only limited powers.
The main opposition party boycotted the February 2 election and protesters disrupted it in parts of Bangkok and the south, the powerbase of the opposition. It may be many months before there is a quorum in parliament to elect a new prime minister.
Demonstrators accuse Yingluck's brother Thaksin of nepotism and corruption and say he used taxpayers' money for populist subsidies and easy loans that have bought him the loyalty of millions in the populous north and northeast.
They want to suspend what they say is a fragile democracy under Thaksin's control and eradicate his influence by altering electoral arrangements.
The government, haunted by memories of a bloody 2010 crackdown by a previous administration that killed dozens of pro-Thaksin "red shirt" activists, has until now largely tried to avoid confrontation.
Tuesday's fatalities brought to 15 the number of people killed in sporadic violence between protesters, security forces and government supporters since the demonstrations began. Hundreds have been hurt.