At least four people were killed and 57 injured as hundreds of Thai riot police tried to clear out anti-government protest sites around Bangkok.

Multiple gunshots were heard near the prime minister's offices, where riot police had started to remove protesters, but it wasn't clear who was firing.

Emergency medical services said one man died from a head wound and a police officer received a fatal chest wound.

It also said another two men died and 57 others were injured.

Special Investigations chief Tharit Pengdit said protesters had launched grenades at police, who later withdrew.

The government announced on Monday that it would reclaim five protest sites around the city under a state of emergency declared in January.

Thousands of police, including armed riot squads, were deployed across the city in operation "Peace for Bangkok."

Meanwhile, the state anti-corruption agency charged Prime Minister Yingluck Shinawatra with improperly handling the failing rice subsidy scheme, putting her in jeopardy of impeachment.

The National Anti-Corruption Commission said the government proceeded with the scheme despite advice from experts that it was potentially wasteful and prone to corruption.

The government has been months late in making payments to farmers for the rice it pledged to sell at above-market prices.

The commission said the prime minister has been called to hear the charges on February 27.

If it decides to submit the case to the senate for possible impeachment, she will immediately be suspended pending trial.