The Eritrean government should release 11 former senior political figures held without trial for almost 10 years, Amnesty International says.
The group, including a former vice-president and two ex-foreign ministers, were arrested after calling for reform.
"We've had no contact since he's been arrested," ex-Foreign Minister Haile Woldetensae's brother told the BBC.
An Eritrean official said the 11 had "committed serious crimes against the national security of Eritrea".
President Isaias Afewerki, who led Eritrea to independence in 1993, has little tolerance for criticism.
Eritrea does not allow opposition parties, independent journalism or civil society organisations.
Amnesty's Eritrea researcher Claire Beston told the BBC's Network Africa the prisoners, who include one woman - Aster Fissehatsion - were all "celebrated veterans" of the independence war and members of the ruling party.
"In May 2001, with four other people, they published an open letter to the president calling for reform, calling for democratic dialogue, rule of law and justice," she said.
"Whilst three of them were out of the country and one withdrew his support, the remaining 11 were arrested on 18 September in 2001 - only later were they accused of committing crimes against the security of the nation."
Michelle Kagari, Amnesty's Africa deputy director, said the anniversary was "a harrowing reminder of President Afewerki's complete disregard for the essential right of freedom of expression".
Eritrean presidential adviser Yemane Gebreab refused to go into details of the case but said the 11 had not been arrested for calling for democratic reforms but for threatening national security.
"If they had succeeded in their plans Eritrea today would no longer exist as an independent sovereign state, or it would have been another Somalia," he told the BBC's Focus on Africa programme.
"Eritrea should be commended for showing restraint on how it has handled this matter."
Haile Woldetensae's brother Daniel said no-one dares to ask the authorities about his brother's whereabouts fearing they too will face detention.
"They've never been tried; they've never had any consular access; no-one has visited them once they were taken from their houses at around 06:30 in the morning - they've never been seen," he said.
He described his brother as "a just an ordinary man, fighter for the liberation of Eritrea".
He said that he had found out from reliable informants that the political prisoners were being kept in solitary confinement in an isolated location.
"We fear the worst - and the president knows that they were psychologically prepared for it and he put them in the conditions that is degrading to human beings, there's no medical treatment and food and everything is very harsh," he told the BBC.
Ms Beston said detainees in Eritrea are often held in underground cells or shipping containers, which are unbearably hot by day and freezing at night..
"The temperatures can be so extreme former detainees have told me that they can't sit or lie down without blistering their skin off their bodies," she said.