Thailand deployed thousands of riot police in Bangkok on Friday to try to reclaim the government headquarters and other areas occupied by protesters seeking to force the prime minister from office.
Thai anti-riot police are seen standing guard near a stadium in Bangkok, on December 27, 2013
The authorities plan to re-take several sites including Government House and the interior ministry, National Security Council chief Paradorn Pattanatabut told AFP, saying officials would try to negotiate with the demonstrators first.
"We will re-take wherever we can and arrest protest leaders," he said. "It's not a crackdown on the protests -- it's enforcement of the law over the protest sites."
He said there was illegal assembly and possession of weapons at the sites.
The operation appeared to be focused on the government district rather than major intersections in the commercial centre that have become the main focus of the rallies in recent weeks as part of what protesters have described as the "Bangkok shutdown".
Riot police removed protesters' tents and warned demonstrators through a loudspeaker not to resist the operation, according to AFP reporters at the scene.
Prime Minister Yingluck Shinawatra's government held a general election earlier this month in an attempt to assuage opposition protesters who have staged more than three months of mass street demonstrations seeking her resignation.
The main opposition Democrat Party boycotted the vote, saying it would not end a political crisis stretching back to a military coup in 2006 that ousted Yingluck's elder brother Thaksin Shinawatra as premier.
Demonstrators prevented 10,000 polling stations from opening in the vote, affecting several million people, mainly in opposition strongholds in Bangkok and the south.
Protesters have occupied major intersections in the capital since January 13, although disruption to people's daily lives has been limited.
Attendance has fallen sharply, with most sites nearly deserted for much of the day and several thousand people joining the rallies in the evenings.
The leaders of the anti-government movement had called for a big two-day protest starting on Friday.
There has been a series of grenade attacks and shootings in the capital -- part of a wave of political violence linked to the protests that has left at least 10 people dead and hundreds injured.