A police helicopter crashed on Saturday night in Rio de Janeiro, killing four officers, after a day of intense firefights with suspected criminals in the slum known as City of God.

It was not clear whether the helicopter was hit by gunfire or whether another factor caused it to crash. Police said they were investigating what happened.

Amateur video sent to local television stations showed the aircraft spinning as it appeared to plummet straight down. The helicopter fell in a populated area near City of God, but crashed in an open space near a major highway.

Crime and violence has been sweeping across Rio, just months after Brazil's second biggest city hosted the Olympics.

An economic recession, rising unemployment and overstretched public finances have emboldened criminal gangs in areas like City of God, one of the best known of the city's so-called favelas, sprawling slums which stretch across vast suburbs and climb up the scenic hillsides of central Rio.

In recent years, police had successfully pushed out drug traffickers and other gangs from many favelas.