Fourteen detained student activists have gone before a military court in Thailand in a case that has attracted international calls for their release and criticism of the military junta's strict controls on freedom of speech and assembly.

The university students, 13 men and one woman, were arrested on June 26 on charges of sedition and violating the junta's ban on political gatherings for leading a peaceful anti-coup rally in Bangkok. Each student faces up to seven years in prison.

They were taken from a Bangkok prison to a military court, which was to decide whether to extend their pre-trial detention for 12 more days. Under Thai law, people can be held a maximum of 48 days without formal charges.

University professors and students joined dozens of supporters to show support for the activists outside the court, across from the Grand Palace in Bangkok's historic quarter, in defiance of the junta's ban on political gatherings. Journalists were not allowed to enter the courtroom.

Thai media have called the case a "hot potato" for the military government, which has come under growing calls both inside and outside Thailand to release the students but does not want to be seen bowing to pressure.

Since overthrowing an elected government in May 2014, the nation's military rulers have jailed opponents who dared to speak out against them.