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 spokesperson 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.
Thailand's Senate was due tomorrow 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.
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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.
"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 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 tomorrow.