At least four people have been killed and dozens injured in violence that erupted as Thai police began clearing protest sites in the capital, Bangkok.
Police were 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.
One of the dead was a police officer - one of about 15,000 that authorities say were involved. Several more were seriously hurt by a grenade apparently thrown by protesters. Police also claimed they were targeted by snipers.
Police too fired live rounds during the operation, which they say successfully reclaimed the first of five sites from protesters, nearly 200 of whom have been arrested.
Demonstrators have occupied official sites over the past few months, calling on the government to step down. The government has announced that it intends 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.
The Erawan Medical Center, which monitors hospitals, said that more than 60 people were injured.
Elsewhere, police reclaimed the besieged Ministry of Energy, with about 100 protesters arrested.
Until now, police had been reluctant to use force against the protesters, allowing 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 Ms Yingluck with improperly handling the government's rice subsidy scheme.
Ms 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% higher than world prices.
The prime minister had been summoned to hear charges on 27 February, the commission added. Reports say she could potentially be impeached and removed from official duties.
Ms 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 Ms Yingluck is controlled by her brother, ousted leader Thaksin Shinawatra.
In response to the protests, Ms Yingluck called snap elections on 2 February, which her government was widely expected to win.
However, the polls were boycotted by the opposition and voting was disrupted by protesters at about 10% 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.