A Thai police officer was killed by a gunshot to the head as authorities launched an operation to clear anti-government protesters from streets in central Bangkok on Tuesday.
Thailand's national police chief also said 14 police were injured.
The protesters have been rallying since November to try to oust Prime Minister Yingluck Shinawatra, whom they view as a proxy for her elder brother, Thaksin Shinawatra, a former premier and telecoms tycoon toppled in a military coup in 2006.
Around 60 people are reportedly injured, with a police officer and a civilian killed in the clashes.
Police are trying to re-take official sites that have been blocked by demonstrators since late last year.
Guns and grenades
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.
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.