The UN Human Rights Office for South East Asia has urged the government to promptly drop criminal charges against the 14 pro-democracy students while the European Union described their arrest and prosecution in a military court as a "disturbing development".
The students were detained on Friday after holding a protest at Bangkok's Democracy Monument the previous day and charged with sedition, a national security offence that carries up to seven years in jail.
Their case is being handled by a military court which usually holds hearings behind closed doors and to which there is no right of appeal once convicted.
The students can be held by police for up to 84 days in pre-trial detention, although their incarceration must be renewed every 12 days by a court with the next hearing at Bangkok's military court expected on July 8.
The junta has defended the move, saying its action is justified based on one of its orders and important to maintain peace.
Authorities also claimed the students were not acting on their own free will but were misled, supported or paid to do so by "a group of people" whom they claim to know but refuse to reveal their identities.
Sirikan Charoensiri, one of a team of lawyers representing the students, said the students were refusing to request bail in protest at being tried in a military court.
"They reject the military court's jurisdiction," she told AFP, adding they have remained "in good spirits" since their arrest.
"It's really quite serious to charge peaceful protesters with sedition. Their activities were entirely peaceful," she added.
Rights groups have described the arrests as a serious escalation in repression by the junta.
Charles Santiago, a Malaysian lawmaker and the chairman of the Asean Parliamentarians for Human Rights, described the arrests as a "disgrace".
"There's no legitimacy left for a regime that bullies and arrests peaceful students for doing nothing more than standing up for their rights," he said in a recent statement.
The generals claim the May 2014 coup was essential to restore order after months of often violent protests against the elected government of Yingluck Shinawatra.
But opponents say it was the latest manoeuvre by Bangkok-based royalist elites, backed by large swathes of the military, to scupper democracy and protect their interests.