Yingluck, Thailand's first female premier, was booted from office by a court days before army chief Prayut Chan-O- Cha seized power in May 2014.
She faces charges of negligence over a multi-billion- dollar rice subsidy scheme which paid farmers up to twice the market rate for their crop.
Critics say the scheme cynically tapped state coffers to prop the Shinawatra's political base, incubated corruption and resulted in massive rice stockpiles.
They gathered in defiance of a ban on political meetings by a junta which has cracked down hard on Shinawatra loyalists.
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Yingluck told supporters she was "confident" of being vindicated by the court.
She was not due to address the court today.
Yingluck has denied any wrongdoing, saying the rice subsidy aimed to help poor farmers after decades of neglect by Bangkok's wealthy elite.
Rice farming communities in the country's poor but populous north and northeast are loyal to the Shinatwara family, who they laud for recognising their changing economic and social aspirations.
That electoral success stirred the Bangkok-centric royalist establishment to sponsor two coups and use their proxies in the courts to remove three Shinawatra-alligned prime ministers.
Fearful of a political comeback, the junta has tried to tangle Yingluck in legal rulings.
She was banned by the military-appointed legislature from politics for five years and is barred from travelling overseas without permission.
Yet Yingluck is still a galvanising force among Shinawatra supporters and remains in Thailand - unlike her older brother Thaksin, a billionaire former premier who lives in self-exile to avoid jail in the kingdom.
"The junta must think carefully of not provoking supporters of Yingluck in the red camp," Pavin Chachavalpongpun, a Thai politics expert at Kyoto University in Japan, told AFP, referring to the "Red Shirt" Shinawatra loyalists.