Cape Town - The police and military routinely torture women, men and children, some as young as 12, human rights watchdog Amnesty International said on Thursday.
Torture has become "institutionalised", with police stations having an informal "officer in charge of torture," Amnesty said in a report released in the capital, Abuja.
"Across the country, the scope and severity of torture inflicted on women, men and children by the authorities supposed to protect them is shocking to even the most hardened human rights observer," said research and advocacy director Netsanet Belay.
Torture techniques include shootings, beatings, nail or tooth extractions, choking, electric shocks, rape and sexual violence.
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Detainees are tortured as punishment, to extort money or to extract "confessions" as a shortcut to "solving" cases, said the organisation, which interviewed hundreds of victims and gathered evidence over 10 years.
"A policewoman ... spread my legs wide and fired tear gas into my vagina ... I was asked to confess that I was an armed robber ... I was bleeding. ... I still feel pain in my womb," Abosede, a 24-year-old torture victim, told Amnesty.
Most of those detained are denied access to the outside world, including lawyers, families and courts.
Medieval witch hunt
The military, too, routinely tortures prisoners, especially detainees suspected to be a member of the terrorist group Boko Haram, according to the report.
The search for Boko Haram fighters had turned into a "medieval witch hunt" that included torture as part of a "screening process", said Belay.
Mahmood, 15, from northern Yobe State, told Amnesty he was arrested by soldiers alongside 50 others last year, detained for three weeks, and beaten repeatedly with the butt of a gun, batons and machetes.
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Soldiers poured melting plastic on his back, made him walk and roll over broken bottles and forced him to watch extra-judicial executions.
Military also arrested and beat a 12-year-old boy in Yobe State, poured alcohol on him, forced him to clean vomit with his bare hands and trod on him, the report found.
Government has refused to properly investigate torture allegations because - although Nigeria's constitution prohibits torture - it is not a criminal offence.
"Parliament must immediately take this long overdue step and pass a law criminalizing torture," said Belay.