In the first six months of 2003, a record 621 civilians were killed by police, said the human rights watchdog - citing official figures.
London-based Amnesty International characterised police behaviour in Rio's poor communities as violent, repressive and often corrupt.
Killings by police routinely went unpunished, it said, and statements by politicians excusing them as an unavoidable part of a crime crackdown gave officers a "green light".
Damian Platt of Amnesty's Brazil team in London said it recognised Rio suffered from endemic crime, but it was still the authorities' responsibility to protect all citizens.
"The situation is extremely complicated and it did appear to get out of control in February," he said.
Research showed many were shot in the back or at close range, he added, indicating summary execution even though official reports registered them as "resisting arrest".
"They always say it's a drug dealer," Platt said.
Though famed for its spectacular setting and beaches, the city of six million is blighted by teeming slums, or favelas, where crime is rampant.
Many areas are controlled by drug gangs that openly challenge authorities by shutting roads, attacking police posts and ordering shops and businesses in tourist areas to close.
But Amnesty said growing numbers of innocent civilians had been killed and police tended to treat everyone in the favelas as potential criminals.
"They always say it's a drug dealer"
"When they come into the favelas, it's a military operation, an invasion. When they talk about security, it's really for the Zona Sul [wealthy, beachfront districts of Ipanema and Copacabana]. In the poor communities, it means an invasion usually resulting in deaths," he said.
History of death
Last year, Rio state police shot dead around 900 people.
Asked to respond to the Amnesty report, a top official in the state Public Security Secretariat, Colonel Jorge da Silva, said: " They didn't speak about the 100 or more policeman killed by bandits in the first half of the year."
He said the problem had to be seen in context. "In a way, the police still don't act as we would like but I can guarantee that it is much better," da Silva said.
Programmes were underway to modernise the police, including giving officers human rights training, he said.
The report was released to mark the 10th anniversary of what are known as the Candelaria and Vigario Geral massacres.
In the first, in July 1993, eight street children and youths were killed as they slept outside a church. A month later, hooded gunmen killed 21 residents of the poor community of Vigario Geral.
Members of Rio's military police force carried out both slaughters. Most remained unpunished and efforts to reform Rio's police had been largely abandoned, Amnesty concluded.