Violence erupted as the riot police tried to take back the protest site at Phan Fah Bridge hours after arresting 100 protesters in front of state-owned oil and gas company PTT Plc for violating a state of emergency declared in January. (Agencies)
Gunfire and explosions broke out after police began the operation called the "Peace for Bangkok Mission".
"Two civilians and a policeman were killed and 59 others were injured in the clash between police and protesters," city's Erawan emergency medical centre said.
Department of Special Investigation chief Tarit Pendith said a policeman was shot in the head and 17 other policemen were injured during the operation. He said the officers were injured by grenades and bullets even though they were using peaceful measures to reclaim the site.
The Centre for Maintaining Peace and Order (CPMO) said it plans to retake Government House, Prime Minister's office, as the first target of its operation to reclaim five areas occupied by the People's Democratic Reform Committee (PDRC).
Labour Minister Chalerm Yoobamrung, the CMPO director, said Prime Minister Yingluck had instructed him not to resort to violence.
The protesters backed by the main opposition Democrat Party have been rallying in Bangkok since November to try to oust Yingluck, whom they view as a proxy for her elder brother Thaksin Shinawatra, a former premier who was toppled in a military coup in 2006.
They want the government to hand over power to an unelected people's council to implement reforms they say are needed to end corruption.
Meanwhile, Thailand's anti-graft panel on Tuesday said that it was pressing charges against Shinawatra for neglect of duty in connection with a controversial rice subsidy scheme.
The National Anti-Corruption Commission said that Yingluck had ignored warnings that the flagship policy was fostering corruption and causing financial losses.
Eleven people have died and hundreds of others injured in political violence linked to the latest round of anti-government rallies. Demonstrators have blocked major intersections and entrances to government offices.
Yingluck's government held a snap election on February 2 which was boycotted by the Democrat Party. Demonstrators prevented 10,000 polling stations from opening on Election Day.
Violence erupted as the riot police tried to take back the protest site at Phan Fah Bridge hours after arresting 100 protesters in front of state-owned oil and gas company PTT Plc for violating a state of emergency declared in January.