The rally was their biggest turnout yet. About 15 km (9 miles) away, in a stadium at the opposite end of the city, about 40,000 pro-government "red shirts" rallied in a show of support of the prime minister. Many came by bus from rural provinces in the north and northeast.
Yingluck has been pilloried by her critics as a puppet for her brother, who was ousted in a 2006 military coup and convicted two years later of graft, which he has denied. He has lived in self-imposed exile since 2008, but exerts enormous influence on the policies of his sister's government.
"We have stood by silently while her brother calls the shots and she runs the country into the ground with loss-making policies," said Suwang Ruangchai, 54, who drove over nine hours from Surat Thani in the south to attend the rally.
Few people in modern Thai history have been as polarising as Thaksin, a billionaire former telecommunications tycoon revered by the poor and reviled by the elite.
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In 2001, he became the first leader in Thai history to win a parliamentary majority on its own, and formed the first elected government to serve a full term, after which it was re-elected. The 2006 coup that ousted him plunged Thailand into four years of sometimes violent political turbulence.
The relative calm Thailand has enjoyed since Yingluck became prime minister has faded during weeks of Democrat-led opposition rallies triggered by a government-backed amnesty bill that could have led to Thaksin's return to Thailand.
The political tensions come as Thailand's economy, Southeast Asia' second biggest, is suffering from weak export growth, soft consumer spending and rising household debt.
"Not yet crisis point"
Demonstrations which began more than three weeks ago have spread even after Thailand's senate rejected the amnesty bill on Nov. 11. Suthep Thaugsuban, a former deputy prime minister under the previous Democrat-led government and now leader of the biggest anti-government rally, has called for all-out regime change.
(Read more: Thailand: the old man of Southeast Asia)
His group plans to march along 12 routes in Bangkok on Monday to urge civil servants to join the protests.
"If even one of you still serves Thaksin, you will have us to reckon with," Suthep told whistle-blowing crowds on Sunday.
Observers say Suthep could be holding out for military or judicial intervention. Thai courts brought down two Thaksin-aligned governments in 2008.
"We have not yet reached crisis point like in 2006 so the military would be unwise to intervene at this juncture and Suthep should know this, but he might be waiting for some form of judicial intervention," said Siripan Nogsuan Sawasdee, a political analyst at Chulalongkorn University.
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Yingluck's ruling Puea Thai Party received a blow last week when the Constitutional Court rejected its proposals to make the Senate fully elected.
That could have strengthened her government given her widespread support among voters in the heavily populated north and northeast.
Her supporters say the verdict is the latest attempt by the elite and anti-Thaksin forces to thwart the legislative process.
The mounting protests are reviving memories of 2010 when thousands of Thaksin's red-shirted supporters stayed in the streets until a military crackdown in which 91 people, mostly red shirts, were killed.
Suthep and former Prime Minister Abhisit Vejjajiva have been charged with murder and accused of allowing troops to open fire with live rounds on protesters.
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Many red shirts loyal to Yingluck say they are prepared to defend the government against political meddling by Bangkok's powerful elite and opposition forces.
"This is the Thai political cycle. Thais from outside of Bangkok vote in a government and the elite in Bangkok kick them out," said Kerk Angchuan, a red shirt protester who joined the pro-government rally in Bangkok on Sunday.