Police in Rio de Janeiro and Sao Paulo have killed over 11,000 people in the past six years, many execution-style, a report claimed.
Few of the officers have been charged over the extrajudicial killings, which are often labelled in Brazilian police reports as the deaths of suspects who resisted arrest, the report by Human Rights Watch said.
The 122-page declaration echoes a 2008 United Nations' finding that police throughout Brazil were responsible for a "significant portion" of 48,000 killings the year before.
"Extrajudicial killing of criminal suspects is not the answer to violent crime," said Jose Miguel Vivanco, Americas director at New York-based Human Rights Watch.
"The residents of Rio and Sao Paulo need more effective policing, not more violence from the police."
Isabel Figueiredo, Brazil's co-ordinator-general of human rights and public safety, admitted police violence was a widespread problem and "it concerns the federal government a great deal".
She said authorities had launched a series of initiatives, including training police to respect human rights and the appropriate use of force, in addition to buying less lethal weapons for state police forces.
Security forces "have begun to understand that instead of solving the problem, confronting criminals with weapons leads to casualties on both sides", she said.
The Rio and Sao Paulo police departments did not comment, but Rio state public safety director Jose Beltrame, in charge of the city's armed security forces, previously took issue with the 2008 UN report.
He said critics did not recognise that his officers constantly confronted drug gangs which ruled over slums and were armed with military rifles, grenades and anti-aircraft weapons.
"We have to deal with something few others face: armed combat with drug traffickers who are equipped with heavy weapons coming from abroad," Mr Beltrame said in October.
He spoke after Rio gangs had unleashed a wave of violence in which they downed a police helicopter, killing three of the six officers aboard - just a mile from the Maracana stadium, where the 2016 Olympics' opening and closing ceremonies and the 2014 World Cup final will be held.
The Human Rights Watch report examined 51 cases in Rio and Sao Paulo in which it seemed that police had killed an alleged criminal, but then reported that the victim died while resisting arrest.
In 33 cases, forensic evidence "was at odds with the official version of what took place" - including 17 cases in which autopsies indicated police shot the person at point-blank range, the report said, adding that "the 51 cases do not represent the totality of potential extrajudicial killings, but are indicative of a much broader problem".
Using government statistics, the report noted that police have killed more than 11,000 people in Sao Paulo and Rio since 2003. In Rio, the killings reached a high of 1,330 in 2007.
The report also states that Rio police killed one person for every 23 people they arrested in 2008, while Sao Paulo police arrested 348 for every person killed.
In the US, police arrested more than 37,000 people for each person killed by police that year.
The report recommends specialised units within state prosecutors' offices to investigate "resistance" killings and ensure officers responsible for extrajudicial executions are brought to justice.