By Gary Duffy
BBC News, Rio de Janeiro
This heavily armed group wears black uniforms and their faces are often masked, and their symbol includes a skull with crossed pistols.
It is not some illegal paramilitary force but an elite battalion of the police in Rio de Janeiro known as Bope, the Battalion for Special Police Operations.
They were created to deal with kidnappings, but their job now is to take on the most dangerous drug gangs in the country, a battle fought with high-calibre weapons in the city's favelas or shanty towns.
Their role has made the force extremely controversial, with accusations that they have killed innocent people whose only crime was to be in the wrong place at the wrong time.
Now the launch of a new movie called Tropa de Elite or Elite Squad has put this police unit in the spotlight in an unprecedented way.
The film is often grim viewing.
It includes scenes in which captives are tortured by police officers to obtain information, and one in which an unarmed suspect is killed. At the same time, a drug gang burns a screaming captive to death, his body wrapped in tyres as he is set alight, apparently to destroy all human remains.
Some police officers are presented as honestly trying to do their job, but many others are portrayed as corrupt.
The movie also points an accusing finger at the hypocrisy of rich young people who complain about violence in Brazilian society, but who use the drugs that finance the gangs which dominate many favelas.
It is estimated that 11 million people saw a pirate copy of the film before it was released.
In a survey in Veja news magazine, 79% of respondents said they believed the film showed the police as they really are, and 72% thought the drug dealers in the movie were treated the way they deserved.
But 51% said torture was not an acceptable way to get a confession from a drug trafficker.
Some critics have accused the film of glamorising violence and making a hero of a police officer who endorses torture and acts outside the law, an interpretation rejected by the film's director.
"What the film does is it exposes violence, it shows violence as it happens, committed by policemen in Rio," Jose Padilha told the BBC News website.
This Bope commander insists his men obey the law
"And in this way it is also a social critique."
Padilha praised Rio's governor Sergio Cabral for removing politicians from the appointment process for local police commanders, but said it was a mistake for the state to fight violence with violence as the costs were too high in Rio.
"The drug dealers are armed to a point, now, where if you go into their favelas to fight them, ordinary people die.
"On the other hand if you don't go, the drug dealers occupy the place and control the place and they become the law there... so it is a very tough situation. It is not an easy decision," he said.
Lead actor Wagner Moura expressed shock at the attitude of some policemen he spoke to while preparing for the film.
"They really think that the solution is to kill the bad guys, kill thieves and it is very scary for me, because they are all good people these policemen. They really think they are doing something good for us."
Fear in the favelas
On the streets of Providencia, a favela whose walls bear the bullet marks of many recent gun battles, there is little sympathy for the police.
We spoke there to Maria da Costa who campaigns with the group Rede contra Violencia, Network against Violence. She said her 19-year-old son was killed by the police.
Favela residents must live through frequent conflict
"They think that whoever comes from a favela is synonymous with a criminal and has to die, has to be punished and eliminated.
"You are totally insecure - not even in your own house are you safe. Anytime a bullet can come inside your home, the police can invade your house, kill your son in front of you, and what counts is the police version," she said.
Maciel Pinheiro de Paula in the favela of Boreu said people feared the police more than the drug dealers, but were too afraid to complain.
"Unfortunately the police nowadays have lost their credibility. If they went as well to the big condominiums they would be fair... but they don't go there, they only go to favelas. They don't go to parties of the elite to arrest people selling drugs."
A short distance away, high in the hills above Rio de Janeiro, officers from Bope receive intensive training.
They often practise their patrols in a nearby favela which is regarded as safe, their black uniforms and high-calibre weapons barely causing a stir among passing residents.
As he watched his men training, one of Bope's senior commanders insisted he did not accept the film's portrayal of police using torture.
The film has provoked debate over how security should be enforced
"The film is a fiction and the author may exaggerate things. The limit of activity of police, in particular Bope, is the law. The law doesn't allow us to do that. Torture is not permissible, torture is a crime," he told the BBC News website.
The commander said any operation in a violent area was difficult as civilian injuries were possible. " We work to try to prevent this, it's not what we want," he said.
But Rodrigo Pimentel, who wrote the film's screenplay, and who served for 12 years in Bope, said torture did happen.
"The Brazilian police tortures and kills - that is a fact. It is part of the daily life of the Brazilian police, " he said.
"For obvious reasons I will not confess to torture. But I would say to you that I used to agree with the actions of the character I created [in the film]. For the first two or three years of my career I believed in this kind of practice. But I left the police condemning those acts."
He said the best legacy of the film was the debate it had provoked among millions of Brazilians over how security is handled.
Without doubt, there is no more important issue confronting this society, but it is clear there are no easy answers as well.