Nigeria is illegally holding hundreds of people suspected of participating in violence perpetrated by the armed group Boko Haram and is denying them access to lawyers, an international rights group has said.
Amnesty International alleged in a report released on Thursday that most of those imprisoned around the country are held without criminal charges.
Some of the suspects in detention have been summarily executed by security forces before facing trial, the London-based Amnesty said.
The group also said some of those detained told its researchers they were shackled for days, forced to sit in their own excrement in overcrowded cells while watching other prisoners get beaten and coerced into confessions.
Boko Haram has been blamed for the killings of hundreds of Nigerians. Many of its attacks have targeted places of worship, often churches.
Amnesty blamed both the Nigerian government and Boko Haram in the report for likely committing crimes against humanity as the guerrilla conflict engulfing the nation's Muslim north continues to kills civilians.
"You cannot protect people by abusing human rights and you cannot achieve security by creating insecurity," Salil Shetty, secretary-general of the group, said at the launch of the report in Nigeria's capital, Abuja.
"There is a vicious cycle of violence currently taking place in Nigeria," the report stated. "The Nigerian people are trapped in the middle."
"We spoke to scores of family members of eyewitnesses, of victims themselves. People who had been attacked by Boko Haram, but also people whose loved ones had been killed by security forces. Eyewitnesses who described they were made to lie down next to someone who was then shot while they were lying down on the ground by a member of the security forces," Lucy Freeman, one of the authors of the report, told Al Jazeera.
Security forces routinely deny committing abuses, though the country has a long history of abuses and so-called extrajudicial killings being carried out by police officers and soldiers.
Colonel Mohammed Yerima, a Nigerian military spokesman, said soldiers do hold prisoners, but only to do a "thorough job" investigating their backgrounds.
He said some people had falsely reported neighbours as Boko Haram members out of petty disputes.
"We don't torture people. We interrogate them and find out if they are members of the Boko Haram," Yerima told The Associated Press. "We don't have any concentration camp that they are talking about. All we have is offices where we work."
Lieutenant Colonel Sagir Musa, a military spokesperson in Maiduguri, also denied the allegations, but said that "lesser human rights infractions and abuses by [Nigerian] troops that do not involve killing are being addressed".
The Amnesty report comes as both Nigeria's government and Boko Haram face increasing international condemnation. Violence blamed on Boko Haram has killed more than 720 people this year, according to an Associated Press count, making 2012 the deadliest year since the group began its attacks in 2009.
A Human Rights Watch report in October also accused Nigerian security forces and Boko Haram of likely committing crimes against humanity in their fighting.
Detainees 'held in slaughterhouse'
The Amnesty report includes claims of killings, house burnings and rapes carried out by security forces, allegations that have trailed the government's response to Boko Haram for months. Amnesty estimates that more than 200 suspected Boko Haram members are being held at a barracks in Maiduguri, while more than 100 others are being held at a police station in Abuja.
Dozens of others probably are being held at the headquarters of the State Security Service, Nigeria's secret police, and others elsewhere, Amnesty said.
Those held largely do not know where they are detained, cannot contact their families or speak to lawyers, in contravention of Nigerian law, Amnesty said.
Many are shackled together for nearly the entire day, the report said.
Those held at the police station in Abuja are kept in a former slaughterhouse where chains still hang from the ceiling, the rights group said.
"There were shots in the night. I was hearing the shot of guns but I didn't know what they are doing," said one former detainee at the police station quoted in the Amnesty report. "When [the police] were collecting statements, some of us cannot speak English, and some of the officers cannot speak our language, so those that have difficulty, they have been beaten ... Our lives were - we were not alive. We had no food, no water and no bath."
Others told Amnesty that soldiers beat at least one prisoner with an electrical cable, while others were denied access to medicine and care. In the report, Amnesty said it requested to see prisons, police stations, military detention centres and holding cells of the Nigeria's secret police, but did not get access to the facilities.
Those arrested by police in Nigeria routinely face years of imprisonment before even being brought to court, due to the country's overburdened judicial system. That has only been exacerbated by the influx of new suspected Boko Haram members, many of whom remain held by a military that does not hand them over to civilian authorities, Amnesty said.
"The failure to prosecute Boko Haram suspects has meant that justice is not being seen to be done, and confidence in the security forces to address the crimes and human rights abuses committed by Boko Haram is being eroded," the report reads.
Boko Haram, whose name means "Western education is sacrilege" in the Hausa language of Nigeria's north, has demanded the release of all its captive members and has called for Islamic law to be implemented across the entire country. The sect has killed both Christians and Muslims in their attacks, as well as soldiers and security forces.
Despite leaders enacting martial law and sending more troops into the region, the group's attacks continue unstopped.
Recently, the military claimed it killed a number of the group's senior leaders and it put out statements claiming to have killed dozens of other members in its operations.
However, some worry those killed by the army include civilians, especially after soldiers in the northeast city of Maiduguri killed at least 32 civilians in a reprisal attack following a suspected Boko Haram bombing last month.
Meanwhile, the killings attributed to the group continue unabated. Attacks by Boko Haram on Tuesday killed at least four people and saw group members burn a police station, a school, a church and a mobile phone tower, witnesses and the military said.