A prominent human-rights leader is still behind bars in Guinea, a week after special service agents detained him. 

Human Rights Watch has called for the immediate release of Mouctar Diallo, a human-rights activist imprisoned in the Guinean capital Conakry.

Human Rights Watch West Africa Researcher Corinne Dufka says the ruling National Council for Democracy and Development wants to silence political opposition.

"We see no reason why he has been picked up.  This is a well-respected individual.  So it appears that this detention - we do not know if he has been arrested, we have not heard that he has actually been charged with anything - appears to really be a campaign of harassment against opposition voices," she said.

Diallo's wife says he was detained a week ago by men from the special service against banditry and drugs.

Security officials in Conakry say he is being held because of an interview he gave to the Voice of America on September 29, the day after soldiers fired at demonstrators protesting against Guinea's military rule.

In that interview, he explained how he came across individuals who had been badly beaten in the September 28 attacks.  He said let them use his cellphone so they could call family members and the Red Cross aid group.

Human-rights groups say at least 157 people were killed on September 28.  The military says 57 people died at the demonstration, which was held in protest of the expected presidential candidacy of ruler Captain Moussa Dadis Camara.

Mouctar Diallo is being held at Conakry's main military barracks Camp Alpha Yaya Diallo.  Corinne Dufka says conditions in Guinea's detainment centers are notoriously bad.

"We spoke with about 16 detainees, or individuals who had been detained in those centers and many of them described very difficult conditions including having been subjected to torture themselves," she said.

Dufka says Diallo's detainment is in direct violation of the law.

"Our reading of the Guinean legal code is that they only have 48 hours to hold him before they are obliged to let him go," she said. "They can appeal through a judicial authority for an additional 48 hours if they believe they still need to conduct investigations, but of course that time now has passed."

Human Rights Watch says neither Diallo or any other detainee was brought before the courts to receive formal charges.

The group is also concerned about Diallo's health.  He is diabetic and suffers from hypertension.  Although he has been allowed to receive food from family members, it is unknown whether he has been granted access to medical care.