At least nine people were killed on Monday in a shoot-out between Brazilian riot police and suspected drug traffickers after a protest in a favela.
The killings at Rio de Janeiro's Maré complex, which lies between the airport and the city on the Avenida Brasil, are likely to prompt fresh concerns about security before next year's World Cup.
The events are disputed. Police claim a peaceful protest turned violent when crack cocaine-users used the disturbance as a pretext to loot a nearby supermarket and rob people.
Heavily armed BOPE police units were called in and a confrontation ensued. After a sergeant was gunned down, a gun battle followed in which two residents and seven drug traffickers were killed, a military police spokesman said.
Residents have given a different version, saying BOPE officers overreacted after the death of their comrade, killing people inside buildings several hours after the first clash.
This is denied by the police. "This was not a case of revenge, it was a police operation to go into the favela to conduct a forensic investigation into the killing," said a spokesman.
Furious locals held a demonstration about the incident on Tuesday, prompting police to send in a tank and hundreds of officers.
Several NGOs operating in the complex have condemned the police action. "Any police operations that leads to the deaths of eight citizens is wrong," said Luke Dowdney, a Briton who established Fight for Peace, a boxing and martial arts centre in Maré. "This war mentality policing is what the state is supposed to be moving away from. We need police to establish a presence here and to bring justice, not the opposite."
Maré is one of Rio's biggest favela complexes, with an estimated 130,000 residents. It is divided into 17 communities, many controlled by different, often-feuding gangs.
Locals told the Guardian that police harassment has intensified in recent months before a planned "pacification" operation, which was expected after the pope's visit to Rio next month.
For the ongoing Confederations Cup, a reinforcement unit from the National Guard was moved to Maré after an engineer from the International Broadcasting Centre was shot when he mistakenly drove into the area after taking a wrong turn on his way back from the airport.
Underscoring problems of violence, gun use and controversial policing, the latest incident will heighten fears for the millions of foreign visitors expected in Rio for three upcoming mega-events: a papal visit, the World Cup and the Olympics.
It will also add to tensions inside Brazil, where more than a million people took to the streets last week to protest against police brutality, corruption, poor public services and over-spending on the World Cup.
Four people died in earlier demonstrations, including three who were run down after blocking the roads and one who had a heart attack after choking in a cloud of police teargas.
