Three people have been killed and dozens injured in violence that erupted as Thai police began clearing protest sites in the capital, Bangkok.
Police are trying to retake official sites that have been blocked by demonstrators since late last year.
Meanwhile, Thailand's anti-corruption body said it would file charges against Prime Minister Yingluck Shinawatra over a controversial rice subsidy scheme.
Thailand has been embroiled in anti-government protests since November.
Demonstrators have occupied official sites over the past few months, calling on the government to step down. The government has announced that it wants to retake all the besieged buildings this week.
The prime minister's office, Government House, has been a focal point for the demonstrators. Thousands gathered outside the building on Monday, cementing the gates shut in a bid to stop officials returning to work.
Early on Tuesday, police started negotiations with the protesters, who over the past few days have come in large numbers to defend protest areas.
Violence then erupted near Democracy Monument in central Bangkok. One police officer and two protesters were shot dead, the Erawan emergency medical services said.
There are also reports of police being injured by grenade attacks and shrapnel from bomb blasts, while police also opened fire on demonstrators.
The Erawan centre said that around 60 people were injured, although it did not report the breakdown between police and protesters.
Elsewhere, police reclaimed the besieged Ministry of Energy, with around 100 protesters arrested.
Until now, police had been reluctant to use force against the protesters. They have previously allowed demonstrators to enter government buildings in a bid to defuse tensions.
'ENEMY OF THE PEOPLE'
Also on Tuesday, Thailand's official anti-corruption commission said it would charge Yingluck with improperly handling the government's rice subsidy scheme.
Yingluck proceeded with the scheme despite warnings it was prone to corruption and could cause losses, the National Anti-Corruption Commission said in a statement.
The programme saw the government buying farmers' crops for the past two years at prices up to 50 percent higher than world prices.
The prime minister had been summoned to hear charges on February 27, the commission added. Reports say she could potentially be impeached and removed from official duties.
Yingluck leads a government that won elections in 2011 with broad support from rural areas.
The anti-government protesters want her to step down, and her government to be replaced by an unelected "people's council" to reform the political system.
They allege that money politics have corrupted Thailand's democracy and that Yingluck is controlled by her brother, ousted leader Thaksin Shinawatra.
In response to the protests, Yingluck called snap elections on February 2, which her government was widely expected to win.
However, the polls were boycotted by the opposition, and voting was disrupted by protesters at around 10 percent of polling stations, meaning by-elections are needed before a government can be formed.
The government is also unable to pay the rice farmers until a new parliament has convened.
On Monday, hundreds of rice farmers gathered in Bangkok, protesting against late payments.