The wave of violence erupted after the murder of a military police officer on Sunday in the northwestern city of Porto Velho, a major state capital with one of the country's highest homicide rates.

In response, police launched an operation against the Comando Vermelho (Red Command) criminal group, which has spread from its base in Rio de Janeiro across the country and even beyond Brazil's border.

The Rondonia state security secretariat said in a report published Thursday that four people were killed in shootouts with police, and 14 people were arrested.

At least seven civilians were killed in retaliatory attacks on the population by Comando Vermelho, the report added.

Brazilian media reported that more than 20 buses and other vehicles had been set ablaze in the state.

Involved in extortion, drug trafficking and other criminal activities, Commando Vermelho has major influence in the Amazonas region.

Earlier in January, a Comando Vermelho boss was killed in a police operation in Porto Velho, local media reported.

Rondonia state forms part of the Brazilian Amazon, where murder rates are 41 percent higher than the national average, according to a study by the Brazilian Forum on Public Security (FBSP) released in December.

The NGO said that criminal gangs now operate in a third of Amazon municipalities -- half of them under the control of Comando Vermelho.

Brazil's other major criminal network, First Capital Command (PCC), is also present in the region, along with other smaller groups fighting for control in more than 80 municipalities.

Organized crime in the Amazon is linked to drug trafficking, but also deforestation and other environmental crimes, said the FBSP.