LAGOS (AFP) – Nigeria's police routinely carry out extrajudicial killings of suspects, torture or rape them while in detention, according to a study released Wednesday by civil rights groups.

The report said the pattern emerging from its study is that the under-funded and ill-equipped police in Africa's most populous country are "more likely to commit crimes than prevent them".

The police "routinely carry out summary executions of persons accused or suspected of crime," said the report by New York-based Open Society Justice Initiative and the Network of Police Reform in Nigeria.

The force also rely on torture as the main method of investigation with every major police station equipped with a "torture chamber".

Rape of arrested females, especially sex workers, has become customary in what one policeman said were "fringe benefits attached to night patrol".

Some female former detainees reported having pepper sprays applied to their genitals while being held by police.

Establishing numbers of suspects executed is "a hopeless task", said the report titled 'Criminal force: torture, abuse and extrajudicial killings by the Nigerian Police Force'.

"There are simply too many, scattered over too large a geographic area," it said.

"Hundreds of Nigerians are murdered each year" by the police, claimed the report, saying people accused of armed robbery stand a higher chance of being executed while in police custody.

Those marked for extrajudicial execution are dubbed "rams" or "bush meat".

Police "commit summary executions, participate in large scale killings and undertake mass burials in shallow graves", it claimed.

"It's almost as if suspected armed robbers in Nigeria are exempt from the constitutional guarantees of life and due process, including the presumption of innocence," said the report.

Beatings, clubbing of soles of the feet, burning of suspects with hot irons or cigarettes, banging their heads against walls and tearing off finger nails are just some of the torture methods regularly applied, claim the groups.

International rights watchdogs have in recent years accused Nigerian police of killing and abusing suspects with impunity.

Police authorities have repeatedly rejected the claims.

But the immediate past Police Affairs Minister Ibrahim Lame in March slammed the "rising cases of extra-judicial killings, human rights violations" as "testimony to the sheer incapacity or wilful defiance of police high command".

President Goodluck Jonathan has said the police must be given "new impetus to perform their duties, even while respecting human rights of Nigerians."

Numerous government appointed panels have in the past recommended an overhaul of the police force, but there is hardly any evidence something is being done.