Thousands of people are protesting in the Thai capital of Bangkok over a new draft political amnesty bill.
Politicians have approved the draft bill that could allow the return of self-exiled former premier Thaksin Shinawatra, one of Thailand's most polarising figures.
Opponents of the bill say it is a thinly veiled attempt by the ruling Puea Thai Party and its leader Prime Minister Yingluck Shinawatra to whitewash the crimes of her billionaire brother.
He was overthrown in a military coup in 2006 and has lived abroad since 2008 to escape corruption charges.
Although it could be weeks before the bill becomes law, the protests in Bangkok threaten to disturb months of relative calm in a country scarred by bloody unrest in 2010.
As well as Mr Shinawatra's opponents, royalist groups and members of the opposition Democrat Party took to the streets.
"Thaksin would not brazenly push this amnesty bill without absolute certainty that it will pass and once that happens, tempers will flare among the anti-Thaksin camp," said Kan Yuenyong, a political analyst at the Siam Intelligence Unit think-tank.
"The situation on the streets could become very volatile indeed."
Mr Shinawatra, who won elections in 2001 and 2005 by landslides, remains a populist hero among the poor, whose votes helped his sister and her party sweep elections in 2011.
A year earlier, Mr Shinawatra's red-shirted supporters paralysed Bangkok demanding the military-backed government hold elections.
The former telecommunications tycoon became the first leader in Thai history to win a parliamentary majority on its own. He formed the first elected government to serve a full term, after which it was re-elected.
But corruption scandals and alleged abuses of power steadily eroded his popularity among Bangkok's middle classes.
That was compounded by royalist accusations that Mr Shinawatra was undermining the country's powerful monarchy, which he denied.
Since fleeing, he has hovered over Thai politics, setting the broad policy lines for the government.