Amnesty outlined the arrest, detention and in some cases the torture of anti-coup dissidents, which "cannot be justified," the group said in a new report.
A further 60 people were facing trial in Thailand before a military court, Amnesty said.
An unnamed military spokesman said the authorities were looking into Amnesty's allegations.
The junta's actions in the days after the coup were taken in order to maintain national security, the spokesman added.
DPA