Tens of thousands of Thai pro-government "Red Shirts" massed in Bangkok on Sunday, police said, in their first show of force since a wave of opposition protests against a controversial political amnesty bill.
Television footage showed a sea of people decked out in red, many bussed in from the country's hardscrabble northeast, at a noisy rally in a suburban park in support of Prime Minister Yingluck Shinawatra and her ruling Puea Thai party.
About 45,000 Red Shirts joined the rally, a spokesman for the national police office said, adding 13,000 anti-government protesters held a separate demonstration elsewhere in the city a few kilometres from Government House.
The major mobilisation of Reds follows several days of protests by various opposition groups against a deeply divisive amnesty bill backed by Yingluck's government, which has inflamed festering political wounds.
Thailand's Senate was due on Monday to debate the bill, which critics say has been crafted to pave the way for a return of the polarising ex-premier Thaksin Shinawatra, who is Yingluck's brother.
The former telecoms tycoon was toppled by royalist generals in 2006 and lives in self-imposed exile to avoid prison for a corruption conviction that he contends was politically motivated.
While the rallies have so far been peaceful, the fear is that the legislation could unleash a fresh bout of political turmoil in a country rocked by a series of rival colour-coded demonstrations since 2006.
Speaking to jubilant crowds, Red Shirt leaders said that the opposition Democrat Party were using the amnesty to try to oust Yingluck's elected government.
"Their opposition to the amnesty bill is just a cover-up... in fact they want to destroy the democratic system," said Worachai Hema, a lawmaker for the ruling party.
"We will not allow anyone to destroy the democratic system -- of which Yingluck is our prime minister."
The bill has however also upset many of Thaksin's supporters, including Red Shirts, who want justice for the killing of more than 90 civilians during a military crackdown on their rallies against the previous Democrat-led government in Bangkok in 2010.
"We disagree with the blanket amnesty bill, which is also an amnesty for murderers," said prominent Red Shirt activist Sombat Boonngamanong, at a separate rally earlier Sunday.
"We want the government to apologise to the people, to the Red Shirts who voted for you" for proposing the blanket amnesty, he added.
Thousands of police have been deployed across the city to keep the peace on Monday.
But there are fears the febrile situation could descend into violence and police banners at the anti-government rally near Government House warned protesters they could face tear gas if they breach security barriers.
Experts say the government may have underestimated the depth of opposition to the amnesty.
Yingluck has appealed for an end to street protests and has indicated the government will not try to ram through the amnesty bill if the Senate rejects it on Monday, as it is expected to do.
The debate will coincide with a scheduled ruling by the International Court of Justice in The Hague on a border dispute between Thailand and Cambodia.
If the ICJ rules against Thailand, the country's opposition -- which includes some hardline nationalists among its supporters -- is likely to try to direct public anger towards the government.