Thailand's senate debated a highly contentious political amnesty bill into the night Monday as tens of thousands of anti-government protesters massed in Bangkok to try to heap pressure on Prime Minister Yingluck Shinawatra.

If, as expected, the senate rejects the legislation, it is likely to embolden a range of anti-government forces who have gathered in large numbers on Bangkok's streets for nearly a fortnight.

Critics say the legislation was crafted to pave the way for the return of the polarising ex-premier Thaksin Shinawatra, who is Yingluck's brother and a hate figure for many among the Bangkok middle and upper classes.

An estimated 50,000 protesters remained in the city's political centre, according to police, heightening fears of clashes with authorities as street politics appeared to return to the capital.

The opposition Democrat Party has harnessed the growing anti-government sentiment.

During an anti-amnesty rally on Monday evening -- before the expected vote by the senate -- one of its most prominent leaders turned up the heat on Yingluck's government by urging supporters to observe a three-day strike starting on Wednesday.

"I ask you to clear all your work tomorrow and then on 13, 14 and 15th (of November) we will stage a nationwide strike," former deputy prime minister Suthep Thaugsuban said to cheers of approval.

Thaksin was toppled by royalist generals in 2006 and lives in exile to avoid prison for a corruption conviction that he contends was politically motivated.

He would be excused of charges under the proposed blanket amnesty.

Suthep, who is facing murder charges related to a bloody crackdown on protests in 2010 when the Democrats were in power, would also be cleared under the proposal alongside his party leader Abhisit Vejjajiva.

But the ruling Puea Thai party last week said it will not seek to ram through the legislation if the upper house rejects it, as predicted by the senate speaker.

"This bill violates the rule of law. All laws must be equal for everyone," said Senator Manoj Kraiwong, echoing the views of several speakers in the televised debate.

Demonstrators, many waving Thai flags, blowing whistles and using plastic hand-clappers, were in a bullish mood before the vote.

"I came here to expel the Shin(awatra) family. I want them out," said one protester called Thamathorn, giving only one name. "Don't stay here and cheat this country. Get out!"

Rallies have so far been peaceful but concerns are mounting that the issue could unleash a fresh bout of political turmoil in a country rocked by a series of rival demonstrations since 2006.

Thousands of police have been deployed to protect Government House -- where Yingluck's offices are -- and parliament in case protests turn ugly, a police spokesman told AFP.

Police have tear gas, batons, rubber bullets and water cannon at their disposal, according to Police General Adul Saengsingkaew, but will only use "necessary force" to ensure a peaceful protest.

The senate debate coincided with a ruling by the International Court of Justice in The Hague which found that most of an area around an ancient temple on the Thai border belongs to Cambodia.

Any public anger in Thailand over the ICJ's decision is likely to be directed against the government by the country's opposition -- which includes some hardline nationalists among its supporters.