Protesters wanted junta leader Moussa Dadis Camara (L) to stand down
Rights groups have called on Guinea's military rulers to charge or free an activist who was arrested last month.
US-based Human Rights Watch says the detention of Mouctar Diallo at an army camp breaks international law.
The group said Mr Diallo was apparently arrested for giving an interview to international media the day after the army suppressed an opposition rally.
The junta has faced bitter criticism over the 28 September crackdown, in which activists say 157 people died.
The government has admitted 58 people died at the protest in the capital, Conakry, but says most were crushed in a stampede.
Activists say the army opened fire on crowds and soldiers took part in gang rapes and murders.
The BBC's Mark Doyle, who was recently in Guinea, says the junta claims to have evidence that Mr Diallo has been involved in anti-state activities.
Our correspondent says the evidence is a low quality, grainy video, which the junta hopes will back up its claims that the 28 September crackdown was in response to some sort of plot against them.
But Human Rights Watch said there seemed to be "no legitimate reason" for his detention.
"This appears to be yet another attempt by the Guinean military government to silence opposition voices," said the group's Corinne Dufka.
"It seems Diallo has done nothing wrong except to speak candidly to the press about a grave episode in Guinea's history.
"If the authorities have evidence he has committed a crime, they should charge him with it; otherwise, they should let him go immediately."
The group said no-one arrested over the protest had been brought before the courts .
Amnesty International has also criticised Mr Diallo's detention and says that arrests, harassment and illegal detentions are continuing in the country.
Amnesty says more than 40 people who attended the 28 September protest are still unaccounted for - and fears they may have been killed or subjected to "enforced disappearances".
The military took power in a bloodless coup after the death of long-term ruler Lansana Conte in December last year.
But the erratic rule of Captain Moussa Dadis Camara has prompted ridicule, protests and international condemnation.