Dozens of people have been injured in violence that erupted as Thai police began clearing protest sites in the capital, Bangkok.
Around 60 people are reportedly injured, with a police officer and a civilian killed in the clashes.
Police are trying to retake official sites that have been blocked by demonstrators since late last year.
Earlier on Tuesday, they arrested around 100 protesters outside a ministry building.
Thailand has been embroiled in a political crisis since anti-government protests began in 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, and one police officer was shot dead, police 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 city's Erawan emergency medical centre said that around 60 people were in.