The suspects, many of them red-shirt supporters, were arrested in the northeastern province a day after the army seized power in a coup on May 22.
The 24 men and two women, ranging in age from 40 to 72 years old, were indicted by a military court in Khon Kaen on Friday, their lawyer Winyat Chatmontree told AFP.
He said the group -- named the "Khon Kaen Model" by the army -- included farmers, a bank security guard, a school director and business owners. Some but not all were affiliated with the red-shirt United Front for Democracy against Dictatorship (UDD), he added.
Most of those arrested were seized in an apartment a day after army chief Prayuth Chan-ocha overthrew the Pheu Thai government.
"The army arrested them and found guns, hand grenades, knives -- things you're not allowed to have without permission," said Winyat. "But [the accused] said other people asked them to keep them."
He said the maximum sentence, if convicted, was the death penalty, although executions are now rare.
At the time of their arrest, the army said the suspects were allegedly intent on a "large-scale attack" in the northeastern province.
The suspects are being held at a civilian prison in Khon Kaen.