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Nigeria's police force carry out extrajudicial killings, torture in custody, and sexual assault against women, according to a study by a civil liberties group.

The report claims that police openly parade suspects for the media, before executing them without trial.

The Open Society Justice Initiative's study is the latest of a number of reports to severely criticise Nigeria's police for brutality and corruption.

The authorities have so far made no comment on the report.

The group observed officers and suspects at 400 different police stations over two years.

They say Nigeria's police, in effect, get away with murder.

"Police in Nigeria commit extrajudicial killings, torture, rape, and extortion with relative impunity," the report says.

"Nigeria Police Force personnel routinely carry out summary executions of persons accused or suspected of crime.

"[The police] rely on torture as a principal means of investigation; commit rape of both sexes, with a particular focus on sex workers; and engage in extortion at nearly every opportunity."

The study details cases of extrajudicial killings and enforced disappearances.

It also describes prostitutes being arrested and raped.

One policeman is quoted saying: "This is one of the fringe benefits attached to night patrol."

Sex workers on the streets of Lagos told the ´óÏó´«Ã½ that the group's report was accurate.

"They pick us, some of them beat us, they will go to third mainland bridge, they will beat us, rape us... They will make love with us without a condom," said one sex worker.

"Sometimes they will search us and steal our money, and then drop us and run away."

Other sex workers describe being attacked by men in police uniform - who appear to come from outside their local area.

There are also accounts of women being forced to use sex to barter their way out of police custody.

In a country where bribes guarantee safety, those who cannot pay are at high risk.

A culture of police impunity is widely criticised - officers are almost never prosecuted for violent crimes.

Several panels on police reform have been set up in recent years and they have made detailed recommendations for improvement, but little has changed.

A review of the Police Act began in 2004, but the draft bill has been pending since 2006.

Nowadays, the police use a slogan to try to soften their image: "Police is your friend."

But few Nigerians believe it.