Thirteen people held for more than 18 years without trial in Eritrea have been released from a notorious military prison, relatives of the detainees have told the BBC.

Among those freed were several prominent figures, including 69-year-old Olympian cyclist and businessman Zeragaber Gebrehiwot.

They had been held at Mai Serwa prison, known for its harsh conditions and where many of the inmates are believed to be political prisoners.

An unnamed source who was once detained in Mai Serwa revealed that the prisoners were arrested in October 2007 following an assassination attempt on Col Simon Gebredingil, a senior internal security officer in President Isaias Afwerki's government.

Initially, around 30 people were detained, and while some have been released over the years, about 20 remained in custody.

Zeragaber competed in the Moscow Olympics in 1980 when Eritrea was part of Ethiopia.

The Eritrean government has not issued any statement regarding the releases. Many of those freed are reported to be sick, which may explain the timing of their release.

The UN and human rights groups have long accused the Eritrean government of serious human rights violations, including torture and forced disappearances. Conditions in Mai Serwa prison have been particularly critiqued, with reports of prisoners held in metal shipping containers and deprived of communication.

Although some have been freed, many detainees and political prisoners remain unaccounted for, sparking ongoing human rights concerns in Eritrea, one of the most repressed nations in the world.