Many young Eritreans have been detained in the capital Asmara after attending the funeral of a man who had died in prison.
The relatives say they are concerned about their wellbeing after being refused any access to visit the prison.
Relatives told the BBC on Thursday that some of the detained Eritreans were as young as 13.
Dozens, possibly hundreds, were loaded onto police vehicles after attending the burial of a 93-year-old man who had died in prison.
"They were taken by three big trucks," a family member of one of those arrested told BBC Tigrinya. Another worried relative said two of their nephews had also been detained.
Musa Mohammed Nur was arrested last year for opposing government regulations at a school where he was a board member.
His detention prompted a rare protest on the streets of Asmara in November.
The Eritrean government has not commented on the arrests.