Nigerian police routinely torture and ill treat suspects, the UN special rapporteur on torture has said.
Mr Nowak said prisoners were tortured to extract confessions
Manfred Nowak said police shot prisoners, beat them and hung them from the ceiling for long periods.
He made his statement after spending a week visiting the country's prisons and other detention facilities.
The police force has denied there is widespread torture in detention cells, saying even if it exists in "isolated cases", it was not official policy.
More than half of Nigeria's prison population has never been convicted of any crime.
It is common for prisoners to wait five to 10 years to come to trial.
At a press conference in the capital, Abuja, Mr Nowak said detainees in Nigerian police cells were "frequently tortured to extract confessions".
He said the most common methods included "flogging with whips, beating with batons and machetes, shooting suspects in the foot, threatening suspects with death and shooting them with power cartridges".
"Suspension (of suspects) from the ceiling or metal rods in various positions and being denied food, water and medical treatment" were also used.
"Torture is an intrinsic part of how law enforcement services operate within the country," he said.
Emmanuel Onwubiko, a senior commissioner with Nigeria's National Human Rights Commission backs the UN envoy and says Mr Nowak's conclusions were "definitely true".
"We receive petitions every day from people who are routinely abused in police detention," he told the BBC's News website.
The Nigerian Police Force welcomed the investigation, but rejected some of its conclusions.
"Definitely we are happy with the report because it takes us closer to a better police force by revealing to us our weakness," national police spokesman Haz Iwendi told the BBC News website.
"But I must say that the allegation of widespread torture is not true because it only existed in the past and the Nigerian police of today is a reformed modern police service."