The government has refused to acknowledge the whereabouts or even the detention of many of them, effectively holding them outside of the protection of the law," the organisations said.
Gambian authorities have detained, and in some cases have "disappeared," friends and relatives of alleged suspects of a failed coup, including elderly women and a child, two international rights organisations said on Wednesday.
Most were arrested in the days following the attempted coup on Dec. 30. Some have not been heard from or seen since they were taken away in early January, Human Rights Watch (HRW) and Amnesty International said in a statement.
"The government has refused to acknowledge the whereabouts or even the detention of many of them, effectively holding them outside of the protection of the law," the organisations said.
"This amounts to enforced disappearance, a serious violation of international law," the statement said, urging the Gambian government to reveal the whereabouts of the detained and either charge or release them immediately.
Gambia officials could not be reached for comment.
About a dozen plotters allegedly headed and bankrolled by U.S.-based housing developer Cherno Njie failed in an attempt to seize the presidential palace in a bloodless coup while President Yahya Jammeh was out of the country.
Njie and another alleged plotter, Papa Faal, 46, a former U.S. Army sergeant, both U.S. citizens with ties to Gambia, have been charged by U.S. federal prosecutors with conspiring to carry out a coup.
Amnesty and Human Rights Watch said one of those being held was 16-year-old Yusupha Lowe, son of Bai Lowe, a man suspected of taking part in the coup but who has fled the country.
The government has refused to provide any information about the boy's whereabouts to family members and there are growing concerns about his safety, HRW and Amnesty said.
Two women in their late 60s, Mariam Njie and Meta Njie, whose sons were killed during the coup attempt, were arrested in early January and their whereabouts are unknown, the rights organisations said.
The U.N. special rapporteur on torture said in March that at least 52 people had been detained in the aftermath of the failed coup. Several have been released but it was unclear how many were still in incommunicado detention, Amnesty and HRW said.
Jammeh, 50, who came to power in a coup some 20 years ago, has stifled dissent in the tiny and impoverished West African nation. He has faced increased criticism from abroad over issues including his human rights record.