Hundreds of people are being held in secret detention in Nigeria, Amnesty International has said.
The rights agency made the allegation on Wednesday at an event to mark the International Day of the Disappeared.
The day is observed across the world every August 30 to draw attention to the fate of individuals imprisoned at places and under poor conditions unknown to their relatives or legal representatives.
Relations of some of the victims of secret detention in Nigeria attended the event on Wednesday at the Nicon Luxury Hotel, Abuja.
"Today is very important for us as Amnesty International because of the work we do to promote and protect human lives.
"And the fact that we want to ensure that victims and victim's voices are heard, it is important for days like this to give them the opportunity to do so," Osai Ojigho, the organisation's Country Director said at the event.
Some families who alleged that their members were suffering arrest or detention by security forces in Nigeria shared their ordeal at the event.
Ibrahim Aliyu, 38, said he had not known the whereabouts or condition of his three younger brothers since 2012 when they were allegedly arrested by the State Security Service, SSS, during a raid at his residence.
"Sometimes in 2012, security agents from the State Security Service arrested my brothers at Gora along Keffi-Abuja highway. They came as early as 2 a.m. in the night.
"Unfortunately for us, the SSS officials stormed my house at the middle of the night and picked them away after jumping my fence, broke the gate and my door. My wife was thinking whether (they were) armed robbers but I told her not to panic because seeing them I noticed that (they were) not armed robbers.
He said he had invited his brothers to his house the day preceding the fateful night so that they could plan their trip to their hometown for Sallah.
"I invited them from their working place to come to my house so that we can know the exact days we can spend in our hometown so that I can write to my office to seek a casual leave to enable us travel for the Sallah," Mr. Aliyu narrated in tears.
"Before my three brothers disappeared, we used to contribute money to support our extended family. Now without them, the burden is entirely on my shoulders. I have to do everything; take care of their families and provide for our mother. Our mother is now perpetually sick, because she thinks a lot about my brothers' fate. Sometimes I feel I can't bear the pain anymore."
Amnesty International said it feared that hundreds of people were being held in secret detention in Nigeria, a conduct it said is prohibited under the International Convention for the Protection of All Persons From Enforced Disappearance.
Saying Nigeria is a signatory to the convention, the group called on Nigerian authorities to release details on the fate and whereabouts of all those who have allegedly disappeared.
"Many families of the victims of enforced disappearance spend painful years searching for justice, truth and reparation but are ignored or misled about the fate of their relatives.
"The authorities must do the right thing now, by releasing all of them or disclosing information about their fate or whereabouts," Mrs. Ojigho said.
The group said a research it conducted revealed that "most enforced disappearances take place in the conflict ridden North-east of Nigeria where young men are often seized by the military after being accused of affiliation to the armed group, Boko Haram."
The organization said it had also obtained details of men, women and children victims of enforced disappearance in other parts of Nigeria.
The country director also highlighted the allegations of human rights abuse levelled against the military during the clash that reportedly led to the killing of over 300 Shiites in Zaria in December 2015.
"According to figures provided by the Islamic Movement of Nigeria, at least 600 of their members' whereabouts is not known since the clashes with the military in December 2015 in Zaria, Kaduna State.
"More than 350 people are believed to have been unlawfully killed by the military between 12 and 14 December 2015," the director stated.
A member of one of the families of the alleged victims of the clash shared her ordeal of years in search for justice.
Zainab Isa said her husband, Abdullahi Abbas, and their six children's whereabouts or fate is not known since the night of December 14, 2015 following the clashes in Zaria.
"He sells books at the Husainiyyah where the clashes took place. All six of our children were with him that day. Up to now, we don't know their fate. We don't know whether the seven of them are alive or dead and no one is giving us any information that can ease our pain," she said.
The director urged the government to probe several cases of enforced disappearance across the country.
"On the International Day of the Disappeared, Amnesty International is calling on the Nigerian authorities to investigate all cases of enforced disappearances and bring all those suspected of criminal responsibility to justice in fair trials before ordinary civilian courts without recourse to death penalty.
"In addition, the authorities must provide full reparation to victims and their families, including compensation, rehabilitation, restitution, satisfaction and guarantees of non-repetition.
"We call on authorities to investigate cases of enforced disappearance across Nigeria to end this crime under international law that makes the victims vulnerable to torture and other human rights violations,"she said.
"The families of the victims of enforced disappearance have already waited too long for answers. They deserve justice, truth and reparation now,"she said.