More than 3,000 police and troops have been mobilized for the rally on the western outskirts of Bangkok, following political violence in which 24 people have been killed and hundreds wounded in recent months. (Agencies)
Thailand has been rocked by years of sometimes bloody street protests by supporters and opponents of fugitive former premier Thaksin Shinawatra, Yingluck's elder brother.
Up to 20,000 Red Shirts were already gathered by today morning, several hours ahead of the official start of the two-day rally, according to Paradorn Pattanatabut, a security adviser to the premier.
"The authorities expect more than 200,000 Red Shirts to turn out," he said, as supporters of the movement poured into the capital in buses and trucks for what is expected to be their biggest show of strength in the months-long crisis.
Paradorn said the authorities did not expect any clashes with rival anti-government protesters who have been holding daily rallies at a park in the city centre, far from the site of the Red Shirt rallies.
"What we are concerned by is third parties," he said, alluding to unidentified assailants who have launched a series of gun and grenade attacks around the capital in recent months, often targeting opposition protesters.
The rival rallies have highlighted the political fault lines that have riven Thai society since a military coup toppled Thaksin in 2006.
Thaksin, a telecoms tycoon-turned-politician, who clashed with Thailand's royalist establishment, has traditionally enjoyed strong support in the northern half of the kingdom.
The ousted premier, who fled overseas in 2008 to avoid jail for a corruption conviction, is hated by many Thais in Bangkok and the south who accuse him of corruption and nepotism.
The opposition says it wants to install an unelected "neutral" leader to oversee vaguely defined reforms aimed at clamping down on corruption and reining in the Shinawatra family's political dominance.
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.
"If they are stubborn and go ahead to appoint a neutral prime minister or stage a coup, the Red Shirts will fiercely oppose it," the movement's chairman, Jatuporn Prompan, told reporters at the rally site.
Yingluck faces neglect of duty charges linked to a loss-making rice subsidy scheme and allegations of abuse of power over the transfer of a top security official.
More than 3,000 police and troops have been mobilized for the rally on the western outskirts of Bangkok, following political violence in which 24 people have been killed and hundreds wounded in recent months.