Twelve people have died and 13 have been arrested since Sunday in violence by drug gangs in Rio de Janeiro, Brazil, military police said Wednesday.
The gangs have shot up a police station and torched at least five buses and 11 cars in the attacks, officials said.
It was not immediately clear if all the dead were gang members, or if civilians and police also suffered fatalities.
The violence stems from the transfer of some 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 those prisoners ordered the violence, Beltrame said.
The attacks also came as a result of police trying to remove some drug gang members from Rio's slums, called favelas.
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.
Brazilian President Luiz Inacio Lula da Silva addressed the seriousness of the situation Tuesday.
"I called the justice minister and told him to attend to Rio de Janeiro with whatever he needs," Lula said on CNN affiliate TV Record.
Beltrame denied reports that two shipments of dynamite stolen recently in Brazil were going to be used in the Rio violence.
The 1,320 pounds (600 kilograms) of dynamite were stolen in Sao Paulo and Rio Grande do Sul. Beltrame said officials who had said the dynamite would be used at public monuments to kill police did a "disservice" to Rio.
Officials are concerned about rampant 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.