President Luiz Inacio Lula da Silva of Brazil traveled to his country's biggest city on Friday to urge an immediate deployment of the army on its streets after weeks of sporadic violence involving a shadowy organized crime group.
Sao Paulo has been the scene of waves of attacks by criminals that have so far claimed more than 200 lives.
The violence, supposedly masterminded by the First Command of the Capital (PCC) group, began on May 12. This week police killed six suspects following three days of attacks across Sao Paulo State, which saw Molotov cocktails and a nailbomb used against public buildings.
So far Sao Paulo's governor, Claudio Lembo, has ignored calls to use federal troops against the group.
"Carrying out attacks has become a tragic and stupid fashion," Lembo said this week. "These are teenagers carrying out small attacks [that are] ridiculous and stupid."
But other prominent figures, including Brazil's former president, Fernando Henrique Cardoso, have begun to predict the beginning of an urban guerrilla war in Sao Paulo.
"It is an unrecognized civil war -- only they are not political groups involved. It is the poor person versus anybody who has something, and that something need not even be very much," said Carlos Amorim, author of The Brotherhood of Crime and an expert on the PCC."This fear is not artificial. People are being terrorized by the violence."
So far the principal victims have been those engaged in fighting crime. Experts, however, fear that government officials and public figures could soon become targets.
"There is no way of controlling it," one prison guard, who refused to be identified, said during a recent visit to the Chacara Belem prison complex in Sao Paulo, a notorious PCC stronghold. "One day someone will just have to come in [to the prison] with explosives and blow the whole thing up."
The guard, who had attended the funeral of one of his colleagues the previous day -- shot by PPC members in Sao Paulo -- added: "We are all completely vulnerable."
Internal memos about further violence have been circulated by prison authorities, adding to the sense of insecurity.
One such note, apparently sent by the state's prison secretariat, detailed the recording of a phone conversation between divisions of the PCC in which members were urged "to break everything without remorse." It recommended "caution -- without panic."
Panic, however, is starting to overcome many of the city's police and prison guards. With thousands of prisoners being allowed out of jail this weekend to celebrate Father's Day, the city is readying itself for a new wave of violence.
The Sao Paulo fire department, in the city center, has parked a fire engine across its entrance to hamper possible attacks, while cordons encircle most police positions.
For an organization that has brought Sao Paulo to its knees on three occasions since May, relatively little is known about the PCC.
The group was founded in the Taubate prison in Sao Paulo in the aftermath of the 1993 Carandiru prison massacre, when 111 inmates were killed by police. It claims to fight "repression in the prison system" and also to be part of a wider "revolution of the poor."