Yingluck arrived at the nation's Constitutional Court Tuesday to face allegations of abuse of power that could see her sacked from office.
The case, one of two potential knockout legal moves against her premiership, comes as Thailand's political crisis reaches a critical juncture.
Hundreds of police, some in riot gear, surrounded the court and searched cars on nearby streets ahead of the premier's arrival at the court.
The premier was due to testify against the complaint, filed by a group of senators who said that the replacement of then-national security chief Thawil Pliensri after she was elected in 2011 was for the benefit of her party.
Under the constitution -- forged after a 2006 coup that ousted Yingluck's billionaire brother Thaksin Shinawatra as premier -- such an offence could lead to her removal and a ban from politics.
End game near?
The Constitutional Court has played a key role in recent chapters of Thai politics.
Critics accuse it of rushing through Yingluck's case and allege previous rulings show that it is politically biased against the Shinawatras.
In 2008, the court forced two Thaksin-linked prime ministers from office.
The backdrop to the current crisis is an eight-year political rupture since Thaksin was booted out of office by an army coup.
The kingdom has become fractured since then, split between the Bangkok-based elites and middle-classes, backed by the royalist south -- and the rural north and northeast and urban poor who have powered Thaksin-led or allied governments to office in every election since 2001.
Six months of political street protests, sparked by a bungled bid to push through an amnesty that could have allowed Thaksin to return, have so far failed to force Yingluck from office.
At least 25 people have been killed and hundreds more wounded in political violence, raising fears of widespread unrest if she is dumped from office.
Observers say the legal challenges appear poised to upend her administration, raising the potential for a backlash by her "Red Shirt" supporters.
Yingluck has also been charged by the National Anti-Corruption Commission (NACC) with neglect of duty in connection with a costly rice subsidy scheme that critics say fomented rampant corruption.
If indicted on those charges, Yingluck would be suspended from office and face an impeachment vote in the upper house of parliament that could lead to a five-year ban from politics.
The Red Shirts have vowed to take to the streets if Yingluck is toppled, raising the specter of clashes with security forces.
"But as of now, it remains impossible to know whether the Red Shirts will offer as much resistance as some of their leaders claim," said Michael Montesano at the Institute of Southeast Asian Studies in Singapore.
The Constitutional Court in March nullified a February general election disrupted by protesters, leaving the kingdom in legislative limbo with only a caretaker government.
Election authorities and the ruling party have agreed on July 20 for new polls, but the date has been rejected by the opposition Democrat Party, casting doubts over its likely success.
AFP , REUTERS