Police have been deployed in Bangkok to retake some sites occupied by protesters since they kicked off their anti-government campaign in November.
Police moved into one of the main roads in the Thai capital's royal quarter, facing little resistance.
But they had to pull back from another protest camp at a government complex because of the likelihood of violence.
The protesters, who want the government to resign, have set up camps at key junctions and government offices.
National Security Council chief Paradorn Pattanatabut said several sites would be targeted.
"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."
The operation was focusing on government sites, rather than the blocked road junctions in commercial and business areas, reports said.
Near Government House, hundreds of police removed barriers blocking a main road, with no immediate opposition from protesters - many of whom had already chosen to leave.
But police had to abandon their plan to retake a larger, well-fortified protest camp in a northern suburb next to several government ministries after protesters refused to move.
To date, security personnel have steered clear of confronting the protesters to avoid violence.
At least 10 people have died since the protesters took to the streets in November. They want the government of Yingluck Shinawatra to resign, saying the prime minister is controlled by her brother, ousted leader Thaksin Shinawatra.
They allege that money politics have corrupted Thailand's democracy and want the government to be replaced by an unelected "people's council" to reform the political system.
In response to the protests, Ms Yingluck - who leads a government that won elections in 2011 with broad support from rural areas - called a snap general election on 2 February.
But the polls were boycotted by the opposition and disrupted by protesters in some places, meaning by-elections need to take place in April before a government can be formed.
The opposition also challenged the legality of the poll, but Thailand's constitutional court rejected the move, citing insufficient grounds.