Carlos Magno de Oliveira Nascimento died where he fell, mercifully quickly. Carlos Alberto da Silva Ferreira's efforts to fend off the fusillade with his forearms were futile; he also died of a gunshot wound to the head. Returning home from work, Everson Goncalves Silote stumbled into the ambush and found himself immediately surrounded by police officers. Witnesses said they saw police shoot him execution-style when he reached for his ID.

Tiago da Costa Correia was the last to die. The gangly 19-year-old ran when he heard the first shots but was struck six times in the chest and abdomen. When the police caught up to him, he was lying fatally wounded on the sidewalk, gasping for air and pleading with them to save his life. "I am a worker," the witnesses recalled him saying again and again. "I have a child."

But the four police officers simply hovered over him, talking among themselves and ignoring him, the witnesses said.

It took 20 minutes for him to die.

"There was no rage," said Leandro de Paula, who saw the shooting spree unfold in April from a distance of about 75 feet. "There was no remorse. There was no shame at what they had just done in plain sight. The police weren't even curious that a man was dying beneath their feet. They were just indifferent, completely indifferent, like they were waiting for a bus.

"They didn't even bother to look down at him."

The killing of four unarmed young men -- a student, a mechanic, a cab driver and a construction worker -- outside a barbershop in this shantytown in northern Rio de Janeiro was part of a growing campaign of terror waged by the police and police-sanctioned death squads, marauding through poor sections of Brazil's cities or attacking street children and in sweeps through drug trafficking areas. The deaths of these innocent victims were uncommon only because witnesses spoke publicly about what they saw.

"The most difficult thing is trying to figure out what to do," said Maria Dalva da Costa Correia da Silva, Correia's mother. "When a crime occurs your first impulse is to call the police. But who do you call when the police are the criminals?"

Across Latin America's largest country -- in shantytowns, or favelas, such as Morro do Borel -- a shadowy network of uniformed, off-duty and retired police officers team with civilians to mete out vigilante justice to drug dealers, petty thieves and other young men, most of whom are black and poor.

A government study this year concluded that death squads operate in 15 of Brazil's 26 states. In Sao Paulo, Brazil's largest city, 435 civilians were killed by police in the first five months of the year, a 51 percent increase over the same period a year ago, according to statistics compiled by Global Justice, an independent Brazilian human rights group.

But it is here in the squalid brick-and-tin favelas that ring the jarringly beautiful coastal city of Rio de Janeiro where the violence is most acute. With more than two months left in the year and a Southern Hemisphere summer approaching, Rio de Janeiro has already equaled last year's record number of police killings. Nine hundred civilians have been killed by police, according to statistics from human rights organizations.

A total of 427 civilians were killed by police in Rio in 1999 and 592 in 2001, according to Amnesty International and the Global Justice center, quoting police statistics. The police classify those killings as "acts of resistance." "Brazil really has a dark side," said Asma Jahangir, the U.N. rapporteur for extrajudicial executions, who visited the country earlier this month to interview witnesses, government officials and human rights activists.

Brazil's militarized state police and local civilian police forces account for more than one in every 10 killings in Rio de Janeiro, according to human rights organizations. Human rights workers and government officials said the problem is likely far worse since official statistics do not include killings by secretive death squads, often staffed and always sanctioned by police to help keep the streets clear of drug dealers.

"This is Brazil's shame," said Luiz Eduardo Soares, the national secretary of public security. "We will not produce a modern state in Brazil until we can deal with the problem of summary executions and vigilante justice by our police."

Poor Brazilians, and especially those descended from African slaves in this former Portuguese colony, have viewed the police as brutal agents of a repressive, wealthy oligarchy. Authoritarian military dictatorships throughout the '60s, '70s and '80s widened the chasm of race and class, academics said.

The two most infamous examples of summary executions by police occurred within days of each other in 1993. In July of that year, six police officers jumped from two cars and opened fire on a group of about 40 street children sleeping in front of the Candelaria Catholic church in central Rio, killing 11 people between the ages of 11 and 22.

The following month, 40 hooded police officers armed with machine guns raided a favela on the outskirts of town known as Vigario Geral in retaliation for the slayings of four police officers. When they finished, 21 people had been killed.

But it is the country's flourishing drug trade that has most profoundly increased friction between police and the poor communities they patrol. Once merely a transit point for illegal drugs produced in Colombia, Bolivia and Peru, Brazil has become the world's second-largest consumer of cocaine after the United States, according to the State Department.

That has transformed Rio de Janeiro's 680 favelas -- home to nearly a quarter of the city's 11.9 million residents -- into a battleground between well-armed rival drug rings and police.

Twenty-two police officers were killed in the line of duty in Rio last year, mostly in confrontations with drug dealers. But that number has dropped slightly for four successive years, even as the number of civilians slain by police has risen dramatically.

A statistical analysis of forensic reports on police killings by Global Justice and Amnesty International indicates that 40 percent of people slain by police last year were shot at close range, 61 percent received at least one shot in the head and for every civilian wounded in confrontations with police, three were killed. A third of all people killed by police showed signs of having been beaten.

"These are executions," said Rubem Cesar Fernandes, director of Viva Rio, a nonprofit organization that campaigns against guns and violence. "These numbers cannot be explained by running gun battles between drug dealers and police and the police come out on top. These numbers can only be explained by torture and summary executions by the police."

In January, a uniformed police officer shot 11-year-old Wallace da Costa Pereira in the back at close range. A 19-year-old police officer on the job for less than a year has been charged with shooting Pereira, who was homeless and bought food with the money he stole picking pockets and committing other petty thefts. Street children who knew him said the police officer had been trying to extort money from him.

In June, residents of Mangueira on the outskirts of Rio said police officers had arrested and handcuffed five suspected drug dealers, then killed each of them with a single gunshot to the head.

Incidents like these horrify residents of poor neighborhoods but comfort them as well, said Fernandes and others. Polls repeatedly show that 15 to 20 percent of Brazilians, weary of crime, support death squads as a way to keep their neighborhoods safe from drug traffickers.

"They only kill the criminals," said Joaquim de Souza, who lives in Rio das Pedras, a poor neighborhood of ramshackle brick homes, open sewage and a death squad that is an open secret. "We don't want the drug traffickers in our neighborhood, selling drugs to our kids. We have poverty, but our streets are safe. Who cares if the rights of some scum are violated?"

Death squads are typically organized by local business people, politicians and police, said Fernandes and Andre Hombrados, a priest at Our Lady of Fatima Catholic Church, which serves Morro do Borel and 12 other poor communities.

"We see all kinds of death squads here," Hombrados said. "There are the death squads that just murder the drug traffickers and the criminals to keep the streets clean. There are the death squads that want to extort money from the drug traffickers and exert some control over the death squads themselves. It's like the mafia, but it's controlled by the police, local politicians and business people, always."

Public ambivalence both enables and encourages vigilantism, according to Fernandes and Soares. Until 1998, police incentives for bravery rewarded officers involved in police shootings with bonuses equal to a month's pay. And crackdowns on crime by politicians have offered tacit support for police aggression, said Timothy Cahill, an Amnesty International researcher. Rosangela Barros Assed Matheus de Oliveira, governor of the state of Rio de Janeiro, told reporters in February, "We don't want anyone to die, but if someone has to die, it is those who are harming society."

Residents of Morro do Borel said the four young men killed by police in April were anything but menacing. None had a criminal record and all either had jobs or were enrolled in school at the time of the shooting.

Nascimento, an 18-year-old exchange student in Switzerland, was visiting his grandmother during a school break. His friend, Correia, 19, left home after working all day to get a haircut and buy chocolate candy for his year-old daughter.

"These young men were as far removed from drug dealers as anyone you could find," said Correia's mother.

The reasons for the killings have never been clear. Police initially said they had discovered drugs and weapons on the four men, who they claimed were drug dealers, but they recanted after witnesses and relatives asked local prosecutors and federal authorities for their version of events that had led up to the deaths. The Morro do Borel Residents Association handed Soares a letter intended for President Luiz Inacio Lula da Silva.

That letter led to a public hearing in May in which investigators reenacted the crime and concluded that the four men were innocent bystanders who walked into a police ambush intended for four drug dealers.

Witnesses said the officers piled the bodies into police cars and took them to the hospital after the shootings even though it was clear that all four men were dead. Residents said that is common. Police officers disturb the crime scene and often dump the bodies in another location, claiming that the suspects died in a shootout with one another, not police, residents said.

"This was a textbook police killing," said Jonas Consalves, president of the Morro do Borel Residents Association. "The only thing different is that this time people had enough and spoke up. These young men were citizens, not criminals. Usually, the only thing that matters is what the police say. This time, we broke the silence."

Five police officers have been charged in connection with the slayings and are awaiting trial.

"The police have terrorized poor people for so long," said Correia's mother, "and people are tired of being afraid. We're not going to allow the police to simply murder people with impunity."

The slayings have showed no signs of abating. Days after Jahangir, the U.N. envoy, left Brazil, three of the witnesses she had interviewed were found dead.