With year-round sun and some of Brazil's best beaches, Recife draws a million foreign tourists a year, many of them on new direct flights from Britain and the rest of Europe. It seems odd then to find an electronic sign in the middle of the city which records the daily murder toll. But behind the narrow stretch of beach restaurants and high-rise apartments shown in the tourist brochures lies a violent city. Nearly 3,000 people were killed in Recife in the past year – up to 12 murders a day - making it Brazil's murder capital. Incredibly many of those who are doing the killing are the police.

So routine is murder in Recife that a small group of residents installed the electronic body count. Eduardo Machado, the group's chief organiser, explained that it was an attempt to shock the city fathers into action because, he claims, at present they are turning a blind eye.

"It's a perverse kind of killing," said Mr Machado. "I call it social cleansing because the people being killed are normally black, they're poor and they're from the slums that surround the city. They have become what I call 'the killables'."

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Many of "the killables" are no more than children who've been driven on to the city's streets by the crushing poverty and violence of their homes in the sprawling slums – or favelas – that stretch back from the city.

A social worker, Demetrios Demetrio, looks after some of these street kids in the city centre. Children as young as 10 sleep rough on the street. The vast majority sniff glue and are high on different types of drugs, including the crack-cocaine that has flooded the favelas in the past seven years. They make their money from begging, stealing and prostitution. Some of them rob the market stalls near where they stay and that makes them a target. "The big danger is getting a death sentence," said one of the street kids, Roberto, 14. "You can also be burned alive here."

According to Mr Demetrio, the biggest threat to these kids is from death squads, made up of local police officers, both former and serving. "They believe they've got to clean up what they see as a social problem by killing these street kids," he said. "Over the years I have personally known 600 street kids killed on the streets – 60 per cent of them have been killed by these organised death squads."

Mr Machado and fellow residents believe the police operating in the death squads are taking the law into their own hands because they feel the justice system isn't working. "The system is just overloaded," he said. "People don't have access to justice. There are not enough judges, not enough attorneys and what happens is, even if the case does go to trial, it can take 10 years."

The head of one of Recife's homicide units, Detective Walcir Martins, admitted that some police are involved in the death squads and estimated that they were responsible for at least a third of city's murders. "It might be hard for them to kill at first but then they get used to it and it becomes an avalanche," he said. "They have no human feeling left."

It took days of persuasion before a death squad member was willing to talk. To avoid spies we met at night at the edge of a favela and drove to a remote beach. He was a police officer and had been in the force for 20 years. He had personally killed more than 30 people, he said, and his "team" had murdered more than 50. He said they killed mainly in the slums. He had a silver handgun in his belt which he took out and carefully ensured it was unloaded before he laid it on the seat between us. "We usually take out rapists and drug dealers and those sort of people," he said. "These are people who, through their actions, require us to perform a service, to get rid of them."

Why did he and police officers like him feel they had to take the law into their own hands?

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"It's right to take a human life in these cases because it takes so long for the legal processes here to go through and the drug trafficker or the killer that we might catch as police officers can be released the next day and go back on the streets and kill and traffic drugs again so it's much better for us to take care of these scumbag crooks, to kill them and solve the problem like that."

He said he felt no remorse because they were performing "a social service". But he admitted they don't just kill to enforce their perception of the law. "The price to have somebody killed would actually depend on type of person you want killed," he said. "It depends, if it's a journalist or a politician or somebody who is just damaging somebody's business, it depends on how powerful that person is."

Senior police officers said they were taking new steps to try to shut down the killing squads, including the arrest of 400 suspects from across the state. But the man we interviewed about this supposed crackdown laughed and said he didn't fear arrest because many of their senior police officers are involved. "Look, it works like this, the senior police officer at a detective or colonel level will call us in for a meeting," he said. "They will say there is a guy we want you to take care of, to kill, we want it done by Friday, we go and do the job, so a lot of police are involved."

The state's security secretary, Servilio Silva de Paiva, said the police and the state government were serious about trying to arrest any police officer involved, but only admitted a limited responsibility for police death squad actions. "If an officer is working on duty as a police officer and he kills somebody then all the responsibility for that lies with us and with me," he said. "If he is off-duty it's not our responsibility."

Recife: Brazil's murder capital

* Travel agents tout Recife as "the Venice of Brazil" thanks to its numerous waterways and bridges.

* A major port on the north-east coast, it is Brazil's fourth biggest city with a population of 1.5 million.

* The city is famed for its beautiful beaches, especially in the Boa Viagem neighbourhood. Porto de Galinhas, 60 kilometres to the south, is known as Brazil's best beach.

* The city's carnival rivals Rio's.

'Unreported World. Brazil: The Killables', Channel 4, 7.35pm tonight