THOUSANDS of protesters have surrounded Thailand's Interior Ministry and forced the evacuation of four others as their campaign to bring down the government of Prime Minister Yingluck Shinawatra intensified.

The demonstrators defied a tough security law imposed late on Monday, after they stormed two other ministries, to control rallies against Ms Yingluck and her billionaire brother, former premier Thaksin Shinawatra.

Ms Yingluck and her ruling Puea Thai Party yesterday began a two-day confidence debate in parliament, where they hold a commanding majority. The opposition has accused them of corruption and trying to pass laws to whitewash Thaksin of a fraud conviction.

Civil servants fled as groups of demonstrators surrounded the interior, agriculture, tourism and transport ministries in blockades that have plunged Thailand into its deepest political uncertainty since it was convulsed by the bloodiest unrest in a generation in 2010.

"Getting rid of the Thaksin regime is not easy," said protest leader Suthep Thaugsuban, a former deputy prime minister under the previous government.

He added that the demonstration "might be longer" than the three days originally planned.

Thaksin remains a populist hero for many, but is reviled by much of the traditional Bangkok elite of generals, royal advisers, middle-class bureaucrats and business leaders who all largely back the opposition Democrat Party.

It is uncertain how long the confrontation will go on. Ms Yingluck might, analysts say, seek to bolster her legitimacy by calling an election she would likely win, as Thaksin has done before, but that could be risky.

"She would first need to be absolutely sure there were no undemocratic forces preparing to fill the power vacuum that would be created," said Kan Yuenyong, director of the Siam Intelligence Unit think-tank.

"Right now, it's brinkmanship. The other side knows an election would only create another Thaksin government. The best scenario for both sides is negotiations, but the problem is they don't trust each other."

After forcing their way inside the Finance Ministry on Monday and bursting through the gates of the Foreign Ministry compound, 3000 protesters circled the Interior Ministry, some wearing plastic bags to protect them from torrential rain.

Staff were ordered to leave five ministries in all and protesters led a march towards the heavily barricaded Government House, Ms Yingluck's offices. However, after a 15-minute standoff with police, the demonstrators withdrew.

A court issued a warrant yesterday for the arrest of Mr Suthep in connection with the raid on the Finance Ministry. Together with former Prime Minister Abhisit Vejjajiva, he has been charged with murder for allowing troops to open fire in 2010 protests by Thaksin's supporters.

Around 90 people were killed in 2010 when Thaksin's Red Shirt supporters occupied parts of Bangkok for weeks before the government, led then by the current opposition, sent in the military. A year later, their Democrat Party was routed in an election that swept Ms Yingluck to power.

The current confrontation is a reminder of the political turmoil that has overshadowed Thailand for much of the last decade.

On one side is Thaksin, a former policeman-turned-businessman from the north who redrew the political map by courting rural voters to gain an unassailable mandate that he then used to advance the interests of major companies, including his own.

On the other is the traditional elite and establishment threatened by his rise, including the royalist military and bureaucracy. Thaksin's opponents include unions and academics who saw him as a corrupt rights abuser, and the urban middle-class who resented their taxes being used as his political war chest.

It is a confusing picture, characterised variously as a class war, a rural-urban split, a clash between ancient and modern or a showdown between royalists and republicans.