Abdi allegedly accused a member of the personal guard of Ismail Omar Guelleh, Djibouti's president, of raping a girl "without checking his sources or consulting those whose reputations were involved".
The activist was arrested on March 9, and was held in prison before being freed on bail last weekend. At the time of his arrest police did not say why he had been taken into custody.
But the International Federation of Human Rights, to which the Djibouti league is affiliated, said police questioned Abdi over his public stance on a common grave that had been discovered a month earlier at Day, in the north of the country.
The grave "contained the remains of seven civilians allegedly executed on January 1, 1994, by the security forces", AFP reported Florence Geel, the Africa bureau programme co-ordinator for the Paris-based rights group, as saying.