It was not immediately clear whether all the dead were gang members or whether civilians and police were among the fatalities.

The continuing unrest stems from the transfer of prisoners from local institutions to federal lockups in other states, the government-run Agencia Brasil news outlet reported, citing Rio de Janeiro Public Security Secretary Jose Mariano Beltrame. Some of the prisoners being transferred ordered the violence by drug gangs, Beltrame said.

Also factoring into the situation was a police effort to remove some drug gang members from Rio's crime-ridden slums, called favelas.

According to the military police, two police officers have been hurt in the unrest, with 66 people arrested and 122 others detained. In addition, 37 vehicles have been torched: 27 cars, two vans, seven buses and a truck, the military police reported. Earlier, officials said the drug gangs also shot up a police station.

At least 47 public schools and 10 nurseries suspended classes on November 25, Agencia Brasil reported.

Beltrame said two rival gangs joined forces to launch the attacks. The security chief also said he mobilized all police in the city to try to restore order and to step up police presence in 17 of Rio's slums.

Officials are concerned about crime in Brazil as the nation prepares to host the 2014 World Cup soccer tournament and the 2016 Summer Olympics, which will be held in Rio.