RIO DE JANEIRO -- Tall and angular, with sad eyes and a warm smile, Mario lives by his wits in a seedy commercial zone. He left home when he was 8 because his parents beat him, he says, and now at 15 he is a veteran of the streets, having survived mostly by begging and robbing. Mario has never been to school.

It is not such a bad life, he says, "except that when it rains, I get cold." Pressed further, he allows that there are other dangers as well.

"A friend of mine was robbing all the time, just robbing and robbing," Mario says, in a voice that betrays neither emotion nor alarm, "so they shot him and he died." He shrugs when asked who "they" might be, then looks up with a puzzled expression.

Mario at least has been lucky enough to find a civic shelter, where he can spend the daylight hours. He exemplifies one of Brazil's most critical social problems -- the hordes of children who live on the streets of the big cities, uneducated, unwashed, unloved.

Estimates of their number range as high as 7 million in a total population of 150 million. Medium-sized urban areas report seeing a sharp increase in street children, but most still live in the metropolitan centers of Rio de Janeiro, Sao Paulo, Recife, Salvador, Belo Horizonte and Brasilia.

New attention was focused on their plight last month when Amnesty International, the human rights lobby, issued a report saying hundreds of street children are being killed each year in Brazil, many by extermination squads who view the slayings they carry out as what amounts to pest control.

"Death squads -- many run by off-duty police officers -- kill them to 'clean up the streets,' remove witnesses or guarantee the security of an area," Amnesty International alleged. The report sparked widespread press coverage and embarrassed the administration of President Fernando Collor de Mello, who has vowed that Brazil must never again be cited for human rights abuses.

The report is based on research that for the most part infers, rather than proves, the involvement of organized death squads. But it provides a context for a steady stream of sensational cases that even a society inured to crime, violence and poverty has found shocking.

In Rio earlier this year, for example, a teenage boy died from a bullet in the brain after being arrested by police and allegedly forced to play Russian roulette. In Sao Paulo last month, a 13-year-old boy was accosted by private security guards who thought he was a graffiti vandal -- they shot him, beat him senseless and then let him bleed to death before finally calling for medical help.

The problem of homeless children is evident in all of Brazil's big cities. By day, groups of barefoot youths play unsupervised in residential streets or saunter through commercial areas looking for robbery victims. Children as young as 3 or 4 dodge traffic at stoplights to beg from motorists. By night, teenagers cruise the docks of Recife, the red-light strips of Sao Paulo and the beachside hotel zones of Rio de Janeiro, selling their bodies.

Social workers say few of these children are true orphans. Most have families and know more or less how to find them, but have been thrown out of the home or have left of their own accord.

"These children come from extreme poverty, and they leave home because they need money," said Vera Moreira, a sociologist who works with the Children's Crusade, an umbrella agency in Rio that coordinates a variety of programs aimed at helping the street children.

"They earn their way by begging, stealing or prostitution," Moreira said. "At first, they leave home to get money to bring home to their parents. Then they decide there's really no need to give the money to anyone, and so they stay out on the streets, on their own."

Some youths sniff glue or use drugs. Some become involved in the violent drug trade that flourishes in the slums, called favelas, sprinkled throughout the big cities. That connection with the drug world has often been cited as a possible factor in the killings.

However, a study by the Brazilian Institute of Social and Economic Analysis -- on which the Amnesty International report was based -- casts doubt on that theory. The institute was able to document 457 slayings of children and teenagers last year in Rio, Sao Paulo and Recife, the three cities where the problem was seen as most acute.

"Most of the victims were boys 15 to 17 years old," said Rosana Heringer, one of the two researchers who conducted the study. "From what we could determine, most had no prior police record. In most cases, there was no apparent link to crime."

The study relied heavily on press accounts, at times sketchy. But Heringer said it appeared that most of the youths were alone or with other youths when they were killed -- no adults were present, a fact that she said tends to refute the notion of young apprentice drug dealers being caught in the middle of shootouts between rival gangs of traffickers.

"Most of the killings were not the result of fights or domestic violence," she said. "Most, it appeared, were premeditated. It is already well documented that there are groups organized in Brazil to kill, to take justice in their own hands. We believe most of these killings fit that pattern."

Volmer do Nascimento, Rio-area director of the National Movement of Street Children, was more blunt in his criticism of the authorities.

"The politicians and especially the police are as guilty as those who actually pull the trigger," he said. "Everybody knows about the death squads. These kids are begging and robbing in front of businesses, and the businessmen want to get rid of them. Then all of a sudden they turn up dead."

Nascimento, whose five-year-old organization has sponsored two national "congresses" of street children and established itself as a leading children's lobby, also was critical of the Collor administration. "We hear speeches but we see no improvement," he said. "On the contrary, crimes against children seem to be rapidly increasing."

The needs of the street children reflect the more general failure of successive Brazilian governments to translate impressive industrial and technological gains into benefits for the vast poor majority. Only two other countries have income distributions more unequal than that of Brazil, according to a recent World Bank study. Political scientists often describe Brazil as not one nation but two -- a small, modern state surrounded by a large preindustrial society; Belgium, say, inside of India.

Most of the homeless children, say social workers, are illiterate. Few have access even to ill-staffed, ill-equipped, state-run health facilities. "The great majority of these children are without opportunities, without a future," said Nascimento.

Activists have begun to try to solve the problem on a local level. The Children's Crusade, for example, coordinates a number of programs in the Rio de Janeiro area where street children are being taught handicrafts. The resultant dolls and wooden toys are sold in upscale shopping centers, with the proceeds going back to the children.

"Perhaps our biggest role is to sensitize the society to the problem," said Children's Crusade director Belmiro Nunes. To that end, the organization is launching programs in Rio's wealthy south zone. The project has the backing of the owners of the major tourist hotels, who have long complained about street children harassing, assaulting and ultimately driving away their clientele.

In the center of the city, Roberto Jose dos Santos heads the Sao Martinho Benevolent Association, where an average of 150 youths -- including Mario, the 15-year-old -- come each day for a few hours' respite from their razor's-edge existence on the streets.

The center, built partly with municipal funds, offers recreational facilities, limited health and educational programs, counseling, legal help and three meals a day. It closes at 5 p.m., however, and the youths return to the streets. Dos Santos said the center eventually wants to open an overnight shelter but lacks the funds.

As it is, however, few of the older boys take full advantage of the center and most remain suspicious.

"They are hard to reach," dos Santos said. "These children already have discovered the adventure of total liberty. And adults? Adults are just people who try to use them."