The protests, now in their third month, have closed off parts of the capital in the latest instalment of an eight-year political conflict that has seen sporadic outbreaks of violence.
They pit the middle class and royalist establishment against the mainly poorer supporters of Yingluck and her brother, ex-premier Thaksin Shinawatra, who was toppled by the military in 2006.
Farmers threaten to join protest
Led by 64-year-old anti-government firebrand Suthep Thaugsuban, the protests were triggered by Yingluck's moves last year to grant amnesty to her brother, the self-exiled former premier Thaksin Shinawatra whom Suthep accuses of nepotism and corruption.
(Read more: As Bangkok 'shutdown' looms, Thailand's 'Teflon' economy put to the test)
Nine people have died since they began in November, the worst violence since 2010. It was Suthep, at that time a deputy prime minister, who sent in troops to end mass protests by pro-Thaksin supporters. More than 90 people died in that unrest.
He is demanding Yingluck step down and a "people's council" be appointed in place. He has given only vague details on the reforms he wants but analysts say his chief aim is to eradicate Thaksin's political influence.
The protesters want to suspend what they say is a democracy commandeered by the self-exiled billionaire Thaksin and alter electoral arrangements so that his allies are unable to return to power.
In a potentially worrying development for Yingluck, whose power base depends heavily on rural support, some farmers have threatened to join the protesters if they do not get paid for the rice they have sold to the state.
A scheme under which farmers are guaranteed an above-market price for their rice has been a centrepiece of the government's programme but, as financing strains mount, some are complaining they have been waiting three or four months to be paid.
The protests are also beginning to undermine Southeast Asia's second-biggest economy.
On Monday, the Thai subsidiary of auto giant Toyota Motor Corp, one of Thailand's biggest foreign investors, said it might reconsider a $600 million spending plan and even cut production if the unrest drags on.
And some economists expect the central bank will be forced to further cut interest rates when it meets on Wednesday to give a lift to the economy.
(Read more: Why Thailand's political strife is far from over)