The criticism comes as security forces, grappling with budget cuts after two years of recession in Latin America's biggest country, also contend with rebounding crime.
In a report to the UN, Amnesty International said that killings by police in Rio de Janeiro had gone up almost 80 percent in the first two months of this year, compared to last year.
Authorities say a cut in funds is partly to blame.
In Rio, where officials on Thursday asked the federal government for about $2.5 million per month to shore up the security budget, criminal gangs are battling for lucrative access to drug routes and sales points.
TRT World's Shamim Chowdhury has more.