Nigerian police say rights group Amnesty treated them unfairly after the group accused them of routinely killing and "disappearing" civilians.
Police spokesman Emmanuel Ojukwu told the BBC that Amnesty's claims would be investigated, but said the group should have come to the police earlier.
He called the police one of Nigeria's "most self-cleansing" bodies and said any guilty officers would be punished.
Analysts say the police are regularly accused of rights abuses and murder.
Mr Ojukwu told the BBC's Focus on Africa programme that Amnesty should have given the police details of the allegations - including names of alleged victims - before its report was published.
When Enugu's Specialist Anti-Robbery Squad rolls out on patrol they look more like a paramilitary gang than the long arm of the law. They zoom through the streets looking down at passers-by, legs dangling from the tailgate of their pickup trucks, arms resting on their automatic weapons.
They are under-trained, under-paid, armed with too much power and not enough official oversight.
Every week police forces across Nigeria parade the armed robbery suspects they have captured. Smiling officers lead young men out of dank cells to talk to journalists. They quietly confess to their crimes.
These young men know that after the reporters leave, there is a good chance they will be taken out to a rough patch of ground and shot.
He said that the police could look after their own affairs without outside interference.
"We are the ones who come out into the open and say what we have done to our own," he said.
"We don't need anybody's prodding to deal with our own who are engaged in misconduct or malfeasance."
And he added that any officer who committed an offence would be "promptly arrested and appropriately dealt with".
Amnesty spent three years unearthing details of prisoners being tortured to death and drivers being shot at roadblocks.
The group concluded that the police were responsible for hundreds of unlawful killings every year.
The publication of the report came a day after a hospital in the eastern city of Enugu told the BBC that the number of bodies brought in by the police was overwhelming.
Andrew Walker, the BBC's former Nigeria reporter, says Nigerians are so familiar with stories of police brutality and extrajudicial killings that such tales barely raise eyebrows any longer.