* One police officer, two protesters killed in gunfire
* Anti-corruption body says PM to hear charges over rice scheme on Feb. 27
* 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
Gun battles erupted between Thai police and anti-government protesters in Bangkok on Tuesday and three people were killed and dozens wounded as authorities made their most determined effort yet to clear demonstrators from the streets.
In a day of tangled developments in Thailand's long-running political crisis, the country's anti-corruption body announced it was filing charges against Prime Minister Yingluck Shinawatra relating to a rice subsidy scheme that has fuelled middle-class opposition to her government.
In a related development, the Government Savings Bank (GSB) said it was calling back a loan to the state farm bank, that manages the rice subsidy scheme, after a high volume of withdrawals by GSB depositors apparently intent on undermining the government's subsidy programme.
The clashes were the most intense in 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, former premier Thaksin Shinawatra.
Reuters witnesses heard gunfire and saw police firing weapons in the area around Phanfa Bridge in the old quarter of the city. Police said they had come under fire from a sniper on a rooftop in the area and that M-79 grenades were also fired.
"One policeman has died and 14 police were injured," national police chief Adul Saengsingkaew told Reuters. "He was shot in the head."
Security officials said separately four police officers had been wounded by shrapnel.
The Erawan Medical Center, which monitors hospitals, said on its website that two protesters, both men aged 52 and 29, had also been killed. The centre said 59 people had been wounded. It did not provide a breakdown of how many of the wounded were police and how many were civilians.
Security officials said earlier that 15,000 officers were involved in the operation, "Peace for Bangkok Mission", to reclaim protest sites around central Bangkok's Government House and other government offices in the north of the capital.
Yingluck has been forced to abandon her offices in Government House by the protesters, who have also blocked major intersections since mid-January.
Police said about 100 protesters had been arrested in an early morning operation to clear demonstrators from another protest site near the Energy Ministry.
Trouble started with clouds of teargas near Government House and soon police were crouching behind riot shields as officers clashed with protesters. It was not clear who had fired the teargas and the authorities blamed protesters.
By mid-afternoon, police had largely withdrawn from protest sites and the streets were quiet.
Thailand's military has remained aloof from the latest crisis but an intensification of the violence would raise the likelihood of the generals feeling compelled to intervene, and possibly removing the government, to end the clashes.
The protesters are trying to oust Yingluck, whom they view as a proxy of Thaksin, a former telecoms tycoon toppled in a military coup in 2006.
Among their grievances is the rice subsidy scheme, a populist move to pay farmers an above-market price that has proved hugely expensive and run into funding problems, and the National Anti-Corruption Commission announced an investigation last month.
On Tuesday, the commission said it was summoning Yingluck to hear charges against her on Feb. 27.
"Although she knew that many people had warned about corruption in the scheme, she still continued with it. That shows her intention to cause losses to the government so we have unanimously agreed to charge her," Vicha Mahakhun, a member of the commission, said in a statement.
The protests have taken a toll on the economy and data published on Monday showed growth slowed sharply in the fourth quarter of 2013. The baht currency weakened after Tuesday's violence.
Yingluck called a snap election in December and has since led a caretaker administration with only limited powers.
The election took place on Feb. 2 but the main opposition party boycotted it and protesters disrupted it in parts of Bangkok and the south, the powerbase of the opposition. It may be many months before there is a quorum in parliament to elect a new prime minister.
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.
They want to suspend what they say is a fragile democracy in the control of Thaksin and eradicate his influence by altering electoral arrangements.
The government, haunted by memories of a bloody 2010 crackdown by a previous administration that killed dozens of pro-Thaksin "red shirt" activists, has until now largely tried to avoid confrontation.
Despite that cautious approach, Tuesday's fatalities brought to 14 the number of people killed in sporadic violence between protesters, security forces and government supporters since the demonstrations began. Hundreds 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."