Her opponents were emboldened by a Constitutional Court decision on Friday to nullify last month's election, delaying the formation of a new administration and leaving Yingluck in charge of a caretaker government with limited powers.
Yingluck's opponents first took to the streets in late November. Twenty-three people were killed and hundreds wounded in the political violence before the protests began to subside earlier this month. But the court ruling appears to have given a second wind to the agitation.
The protests are the latest instalment of an eight-year political battle broadly pitting the Bangkok middle class and royalist establishment against the mostly rural supporters of Yingluck and her billionaire brother, former premier Thaksin Shinawatra, who was ousted in a 2006 coup.
There are growing fears that Thailand could be heading towards serious civil unrest. After months of restraint, Thaksin's "red shirt" supporters have begun making militant noises under hardline new leaders.
They plan a big rally on April 5, possibly in Bangkok, and the political atmosphere is expected to become even more highly charged in coming days.
Yingluk has until March 31 to defend herself before the National Anti-Corruption Commission for dereliction of duty over a ruinous rice-buying scheme that has run up huge losses.
If the commission recommends her impeachment, she could be removed from office by the upper house Senate, which is likely to have an anti-Thaksin majority after an election for half its members on March 30.