Thousands of riot police were deployed in the Thai capital on Friday to clear areas occupied for weeks by opposition protesters seeking to force Prime Minister Yingluck Shinawatra from office.

The operation marked an unexpected shift in tactics by the embattled government, which has allowed the protesters to camp out at locations around Bangkok.

Police with shields and riot helmets, some carrying rifles, met little resistance as they re-took areas around Government House, which Yingluck had been unable to use for about two months, according to AFP reporters.

Security forces removed protesters' tents and tore down their makeshift defences built from barbed wire, sandbags and piles of rubber tyres. Police warned demonstrators through a loudspeaker not to resist.

But it was unclear if the operation was a success as demonstrators were later seen rebuilding barricades after police withdrew.

Labour Minister Chalerm Yubamrung, who oversaw the operation, said officials would return to work at Government House on Monday.

He said sling-shots, illegal drugs and bomb-making materials were discovered at the rally site.

"Protesters -- you should return home," Chalerm said in a televised national address from the government headquarters.

"If you're still stubborn we will gently enforce the law," he added. "The police are ready to disperse protesters but the prime minister told us not to use force to avoid loss of life."

There were no reports of serious clashes. Two people were slightly injured by what appeared to be a firecracker during a brief confrontation between police and protesters.

The security operation was 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".

So far the authorities have not announced any plan to clear those intersections, where hundreds of protesters are gathered. The leaders of the anti-government movement had called for a big two-day rally starting on Friday.

But there are plans to re-take several other sites including the interior ministry, National Security Council chief Paradorn Pattanatabut told AFP, saying officials would attempt 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."

Yingluck'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 demanding her resignation.

The protesters want Yingluck to stand down to make way for an unelected "People's Council" to enact reforms to tackle corruption and alleged vote-buying before new polls are held.

The main opposition Democrat Party boycotted the February 2 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.

As part of their rallies, 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.

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.

Yingluck's opponents say her government is controlled by Thaksin, who fled Thailand in 2008 to avoid going to jail for a corruption conviction and now lives in Dubai.

Pro-Thaksin parties have won every election for more than a decade, most recently in 2011 under Yingluck, helped by strong support in the northern half of the kingdom.

The Election Commission on Tuesday set a date of April 27 for election re-runs in constituencies where voting was disrupted by protesters. But there is still no decision on what to do about 28 constituencies that have no candidates.

On Wednesday, in a boost to the government, the opposition lost a legal bid to nullify the February election.

Yingluck also faces an investigation by an anti-corruption panel into possible negligence of duty in connection with her flagship rice subsidy scheme.