* Police officer killed in gunfire
* Teargas fired near Government House
* Police operation involves 15,000 officers; 100 protesters arrested
* First of five sites reclaimed from protesters, police say
* "Reforms will benefit your children, too," protest leader tells police (Updates with police officer killed, numbers wounded)
By Athit Perawongmetha and Damir Sagolj
BANGKOK, Feb 18 (Reuters) - A Thai police officer was killed and dozens of police and anti-government protesters were wounded in gun battles and clashes in Bangkok on Tuesday, officials and witnesses said.
Violence erupted as the authorities launched their most determined effort yet to clear demonstrators from sites around state offices in the capital, where anti-government rallies have been taking place since November.
"One policeman has died and 14 police were injured," national police chief Adul Saengsingkaew told Reuters. "The policeman ... died while being sent to hospital. He was shot in the head."
Three Reuters witnesses heard gunfire and one saw police firing weapons in the area around Phanfa Bridge in the old quarter of the city. Security officials said that four police officers had been wounded by bomb shrapnel.
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.
Data published on Monday showed growth slowed sharply in the fourth quarter of 2013 as the protests took their toll on the economy, and the baht currency weakened after Tuesday's violence.
The protests are the latest instalment of an eight-year political battle broadly pitting the Bangkok middle class and royalist establishment against the poorer, mostly rural supporters of Yingluck and her billionaire brother Thaksin.
Demonstrators accuse Thaksin of nepotism and corruption and say he used taxpayers' money for populist subsidies and easy loans that have bought him the loyalty of millions in the populous north and northeast.
The government's opponents want to suspend what they say is a fragile democracy in the control of Thaksin and eradicate his influence by altering electoral arrangements in ways they have not spelt out.
Security officials said 15,000 officers were involved in an operation, called the "Peace for Bangkok Mission", to reclaim protest sites around government offices in the centre and north of the capital.
Television pictures showed clouds of teargas and police crouching behind riot shields as officers clashed with protesters near Government House. It was not clear who had fired the teargas and the authorities blamed protesters.
"I can guarantee that teargas was not used by security forces. The forces did not take teargas with them," National Security Council Chief Paradorn Pattanathabutr told Reuters. "Protesters are the ones who threw teargas at the security forces."
The Erawan Medical Center, which monitors Bangkok hospitals, said 44 people were hurt. It did not provide a breakdown of how many of the wounded were police and how many were civilians.
Live television pictures showed police with shields and batons pushing and jostling with protesters near Government House. One man could be seen bleeding from a head wound.
Police said about 100 protesters had been arrested in an early morning operation to clear demonstrators from another protest site near the Energy Ministry.
Yingluck has been forced to abandon her offices in Government House by the protesters, who have also blocked major intersections since mid-January.
The government, haunted by memories of a bloody 2010 crackdown by a previous administration that killed dozens of pro-Thaksin "red shirt" activists, has largely tried to avoid confrontation.
Despite that cautious approach, Tuesday's fatality brought to 12 the number of people killed in sporadic violence between protesters, security forces and government supporters since the demonstrations began. Hundreds more have been hurt.
Bluesky TV, the protest movement's television channel, had earlier shown protest leader Suthep Thaugsuban addressing police lines near Government House.
"We are not fighting to get power for ourselves," Suthep said. "The reforms we will set in motion will benefit your children and grandchildren, too. The only enemy of the people is the Thaksin regime."
Labour Minister Chalerm Yoobamrung, who is in charge of the security operation, has said police would reclaim sites near Government House, the Interior Ministry and a government administration complex in north Bangkok as well as the Energy Ministry.
"We will not respond with force. We will not give up Government House and the Interior Ministry," said protest spokesman Akanat Promphan.
Police have made no move against the largest protest sites in the city's main business and shopping districts. (Addtional reporting by Apornrath Phoonphongphiphat, Amy Sawitta Lefevre, Prapan Chankaew and Panarat Thepgumpanat; Writing by Alex Richardson; Editing by Alan Raybould and Robert Birsel)