Thai authorities along the country's northwestern border braced themselves Monday for a possible influx of more ethnic Karen villagers fleeing new airstrikes from the Myanmar military.
The prisoners appear to be the hundreds of students detained in early March while demonstrating against the February 1 coup that ousted the elected government of Aung San Suu Kyi.
The military junta warned it was prepared to use lethal force against rallies inciting "riot and anarchy".
Street protests against last week's military coup continued on Wednesday.
The coup came earlier this week as the country's new parliament was set to sit for the first time following elections last year.
People waited until 8 pm (local time) when they began hitting pots and pans on the street.