Amnesty International says security groups linked to the president of the Democratic Republic of Congo are guilty of human rights abuses.
The human rights organisation says the Presidential Guard and the Special Services Police committed murder, rape and torture with impunity.
Most of the abuses took place after last year's presidential elections.
The government said then it had to deal firmly with widespread insecurity caused by armed opposition supporters.
The BBC's world affairs correspondent Mark Doyle says the immediate post electoral period in DR Congo, at the end of last year and the beginning of this, was fraught with tension.
In its report Government-backed agents of torture and death in DRC, Amnesty notes that there was a climate of violence, with armed clashes between groups loyal to President Joseph Kabila and supporters of the defeated opposition candidate, Jean-Pierre Bemba – who was contesting the result of the presidential election.
Amnesty notes that Mr Bemba's men were also accused of abuses, but his forces were defeated militarily and subsequently disbanded.
The report concentrates on abuses by the Presidential Guard and the Special Services Police, both closely connected with President Kabila, and calls on him to bring them to justice.
Amnesty says there are credible allegations of extrajudicial executions at the Republican Guards' base in Camp Tshatshi on the banks of the River Congo.
It says bodies were found in the rapids a short distance downstream from the camp.
The report also documents the leading role taken by the Special Services Police in the arrest and torture of scores of supposed political opponents of President Kabila's ruling party.
"People have been targeted by the security forces simply because they share the same ethnicity as Jean-Pierre Bemba," said Erwin van der Borght, director of Amnesty's Africa programme.
The report says some of these people are still being held, including one woman who says she was raped by numerous policemen and requires medical attention.
Amnesty concludes that far from protecting the people of DR Congo, the state security services remain agents of torture and death.
Mr van der Borght said it was crucial to reform the police and he called on Mr Kabila "to ensure all government and armed opposition forces are integrated into one politically-neutral and accountable entity that operates within Congolese and international human rights law".
The report has been published as President Kabila undertakes an official visit to the United States, where he is expected to have a meeting on Friday with President George Bush.