The demonstrators, a motley collection aligned with Bangkok's royalist civilian and military elite, accuse Yingluck of being an illegitimate proxy for her billionaire brother, former Prime Minister Thaksin Shinawatra, a populist hero of the rural poor who was ousted in a 2006 military coup.
Most of the 31 provinces where demonstrators had massed are in the south, a traditional stronghold of the opposition Democrat Party, although four were in the north and northeast, where the Shinawatra family is hugely popular.
The aim of the rallies was to wipe out the "political machine of Thaksin", said Suthep, a former deputy prime minister under the military-backed government that was routed by Yingluck in a 2011 election.
(Read more: Thai capital hit by biggest protests since deadly 2010 unrest)
Amnesty bill sparked protests
The protests are all-too familiar in Thailand, which has seen eight years of on-off turmoil, from crippling street rallies to controversial judicial rulings and army intervention, each time with Thaksin at the centre of the tumult.
Despite fleeing into exile to dodge a jail sentence for abuse of power in 2008, billionaire former telecommunications mogul Thaksin has loomed large over Thai politics.
He won the support of the rural poor who voted him twice into office, in 2001 and 2005, before he was ousted in a 2006 coup. His supporters remain fiercely loyal to him and swept Yingluck to power in an election landslide in 2011.
Thaksin's opponents are fewer in number than his supporters but hold considerable power and influence, among them wealthy conservatives, top generals, bureaucrats, royalists and many members of the urban middle class.
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Many of them see Thaksin as a corrupt, crony capitalist who manipulates the masses with populist handouts and is a threat to the monarchy, which he denies.
The anti-government campaign started last month after Yingluck's ruling Puea Thai Party tried to pass an amnesty bill that critics said was designed to absolve Thaksin of his 2008 graft conviction.
The protests, though peaceful, have raised fears of unrest. Anti-government protest leaders, from all sides, have a tradition in Thailand of trying to provoke a violent crackdown by the government to rob it of legitimacy.
Fearing clashes could erupt and further weaken her government, Yingluck said police would keep the peace.