Thailand's pro-government "Red Shirt" supporters rallied for a second day Sunday, vowing to protect Prime Minister Yingluck Shinawatra, who is facing a slew of legal challenges that could see her toppled within weeks.

Tens of thousands of Red Shirts descended on a wide road in a Bangkok suburb in a colourful and boisterous show of support for the crisis-mired premier, who has faced months of anti-government demonstrations in the Thai capital.

Drawn mostly from the poor but populous north and northeast, the Red Shirts say they will not accept the removal of another democratically elected government by a Bangkok-based royalist establishment backed by the judiciary and the military.

"This will be the final fight," Red Shirt chairman Jatuporn Prompan told the rally, which is due to end on Monday.

"We are here to settle the bill with the elite," he said, adding the Red Shirts were would not be deterred by any crackdown on their movement.

"It is better to die than to be slaves," he said.

Scores of people died in 2010 in an army crackdown on a Red Shirt rally against the ousting of a Thaksin-allied government by the nation's courts.

Political violence linked to the current round of turmoil has killed 24 people and left hundreds wounded, raising fears of a wider civil conflict if the two bitterly divided sides can not reach a compromise.

That seems unlikely with kingdom's political crisis poised to enter a new and potentially turbulent phase with legal challenges mounting against Yingluck.

By Sunday afternoon "there were an estimated 30,000 Red Shirts," said Paradorn Pattanatabut, a security adviser to the premier, told AFP.

He said the rally drew around 100,000 people at its peak on Saturday evening, several times larger than a figure given by the rival anti-government movement.

Opposition protesters in Bangkok want to oust Yingluck and install an unelected premier to oversee reforms aimed at curbing corruption and money politics, which they blame on the rise of billionaire Thaksin Shinawatra.

Thaksin, Yingluck's elder brother, became premier in 2001 before being ousted in a military coup five years later and lives overseas to avoid jail for a corruption conviction he says was politically motivated.

Yingluck could face indictment over negligence charges linked to a controversial and costly rice subsidy scheme.

The nation's Constitutional Court is also mulling accusations of abuse of power by the premier over the transfer of a top security official.

The Red Shirts accuse the courts of political bias and have pledged to resist legal moves to topple Yingluck, which are expected within weeks.

The rival rallies have highlighted the political fault lines that have riven Thai society since Thaksin's overthrow by royalist generals in 2006.