Amnesty International accused a Nigerian police unit on Wednesday of torturing suspects and demanding bribes to free them - allegations dismissed by the police.
The rights group said people arrested by the Special Anti-Robbery Squad (SARS) were starved, beaten, shot and subjected to mock executions "until they either make a 'confession' or pay officers a bribe to be released."
Nigeria's national police force denied the allegations, saying it did "not employ torture or any form of unauthorized technique in criminal investigation".
"Investigators are trained to conform to international best practices," said spokesman Don Awunah.
A 32-year-old man, Chidi Oluchi, told Amnesty he was tortured after being arrested by SARS officers in the southeastern city of Enugu.
"They started beating me with the side of their machetes and heavy sticks. My mouth was bleeding and my vision became blurred," he was quoted as saying in the report, adding that he was released after paying SARS officers 25,500 naira ($80).
Amnesty said it was told by a senior officer that around 40 officers accused of manhandling detainees had been transferred to other stations in April 2016, although he did not say whether the claims against them had been investigated.
"It is time for the authorities to ensure that officers responsible for such human rights violations are finally held accountable," said Amnesty's Nigeria researcher, Damian Ugwu.
"There is also an urgent need for robust legislation that ensures all acts of torture are offences under Nigeria's criminal law."