President Luiz Inacio Lula da Silva of Brazil travelled to his country's biggest city yesterday to urge an immediate deployment of the army on its streets after weeks of sporadic violence involving a shadowy organised 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," Mr Lembo told reporters 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 unrecognised 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 terrorised 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, told the Guardian 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.

The Guardian had access to one such note, apparently sent by the state's prison secretariat. It 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 centre, has parked a fire engine across its entrance to hamper possible attacks, while cordons encircle most police positions.

For an organisation 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".

According to Mr Amorim, one of the PCC's key strengths is its capacity to recruit "freelance" members to carry out specific actions. Many see involvement as a means of self-protection. "It is a kind of seguro-cadeia [jail insurance]," Mr Amorim said. "You know you will be arrested one day, so it is better to arrive in prison as a friend or at least someone who is known [to the PCC]."

Propaganda, and the dissemination of fear, appear to be increasingly important parts of the group's armoury.

On Thursday, the Folha de Sao Paulo newspaper reported receiving a video - supposedly produced by the PCC - featuring a masked man flanked by machine guns and sticks of dynamite. In the video, he reportedly made demands in the name of the "party", saying at one point: "Don't mess with our families and we won't mess with yours." Threats of violence tomorrow have also been circulated via the internet.