'Democracy will prevail'
Meanwhile, Ms Yingluck's trial on charges of dereliction of duty over her role in a controversial rice subsidy scheme has started. She pleaded not guilty.
The scheme paid rice famers in rural areas - where her party has most of its support - twice the market rate for their crop, in a programme that cost the government billions of dollars.
Ms Yingluck says she was not involved in the scheme's day-to-day operations and has defended it as an attempt to support the rural poor.
In January, she was retroactively impeached for her role in the scheme. She was also banned from politics for five years.
She told crowds outside the court in Bangkok she would prove her innocence.
Ms Yingluck maintains the charges she faces are intended to keep her out of politics. The next hearing in the trial has been scheduled for 21 July.
Her brother, Mr Shinawatra made a rare public appearance in South Korea earlier saying he believed "democracy will prevail" in Thailand.