Human Rights Watch has accused Bahrain's police of continuing to beat and torture detainees, including minors.
The report comes nearly six months after an independent inquiry prompted the government to pledge reforms.
The country insists it is committed to putting the recommendations of its own report into the handling of protests in 2011 into practice.
More than 40 people died in last year's unrest and 1,600 were arrested.
According to the Human Rights Watch (HRW) report policemen regularly take young men to secluded places and beat them for up to two hours before transferring them to a police station.
Some said they had been threatened with rape if they did not reveal where activists were hiding the petrol bombs that are regularly hurled at police.
The report said treatment inside police stations had improved significantly in the last six months, but it also warned that unlawful police behaviour on the streets may well make young protesters even more desperate and determined to confront their government.
The campaigning group said it had interviewed 14 young males, including seven children. It said five of the beatings had happened in April alone.
"Bahrain has displaced the problem of torture and police brutality from inside police stations to the point of arrest and transfer to police stations," said Nadim Houry, deputy Middle East director at HRW.
On Sunday, there were reports that a 13-year-old was being held in custody for assaulting a police officer and taking part in a street gathering in a village south of the capital, Manama.
According to his lawyers, quoted by AFP news agency, the boy was "beaten and tortured" at a police station where he was still being held.
The HRW report comes a few weeks after Amnesty International also warned that the country's reforms had only scratched the surface.
In November, the Bahrain Independent Commission of Inquiry (BICI) report acknowledged numerous human rights abuses and systematic torture of detainees as security forces put down anti-government protests.
In response, King Hamad promised lessons would be learned and laws would be reformed to protect freedom of speech and other basic rights.