Taizo Sudo, an ailing, jobless drifter, had hunkered against an open-air food vendor's stall in a deserted waterfront park here for shelter against the icy, evening breeze. Suddenly, a band of youths, some wielding shovels, swooped down on the 60-year-old vagrant, beating and kicking him into a coma.

Then the attackers, their jet-black hair done up in duck-tail fashion and tinted in shades of orange and blond, hurled the fatally injured Sudo into a bamboo trash container, spinning him around the park.

The macabre nighttime lark was part of a wave of juvenile terror that has rocked this bustling port city 30 miles south of Tokyo since the beginning of the year. It has left three men dead and 13 others seriously injured, and it has spawned a sweeping investigation by police that has brought the arrests of 10 teen-agers.

While acts of youthful violence long have been a painful fact of life in most big U.S. cities, they are particularly shocking to parents, educators and police in this orderly country of 117 million where centuries-old Confucian traditions of respect for authority figures and the aged have only recently shown signs of fraying around the edges.

The outbreak of "hobo-hunting," as the press here refers to it, generally fits a mushrooming pattern of violent behavior among 13-, 14- and 15-year-olds, mostly operating in gangs, at junior high schools throughout Japan. It features increasingly frequent, brutal classroom attacks on teachers with clubs, fists and zip guns. To many Japanese adults, these occurrences represent a rent in the country's tightly woven social fabric through which they glimpse a chilling new world of alienated youth and random violence.

"What happened in Yokohama," said Kaichi Kanazawa, a respected authority on education and juvenile problems, "is typical of a spreading social illness." While the growth of Japan's economy since World War II has fostered an unprecedented degree of material prosperity, he explained, it also has helped promote a new permissiveness among Japanese parents who are less apt to enforce the old ironclad family disciplines on their children.

Add to this violence on television and in comic books, the greater mobility and anonymity of urban living that has helped destroy previously cozy neighorhood ties, the influence of older boys who have joined Japan's many criminal and hot-rodders' gangs, and you have a recipe for an upsurge in juvenile delinquency that has baffled and horrified the Japanese.

In 1982, the police took into custody 1,900 students for abusing teachers, a six-fold increase from 1978, when law enforcement authorities first felt compelled to gather statistics on such cases. Almost 95 percent of the offenders are in junior high school and most are boys. Officials say many more incidents of aggravated assault, especially those against fellow students, go unreported because of fear of reprisals.

Police and educators stress that the vast majority of Japan's junior high school students are well behaved, respectful of teachers and parents and toe the line where strict campus codes governing dress and conduct are concerned. But, at the same time, increasingly fierce competition for grades and high exam scores needed to enter the country's elite universities has created a large pool of frustrated youths who cannot pass muster.

In Japan's rigid social pecking order, the gates of college are open only to those who have successfully passed through "examination hell," the torturous battery of testing for university placement. The lack of a college degree, in effect, slams shut the door to lucrative, high-status jobs in business and government and diminishes the prospects for marrying into a higher social station. Junior high school is a watershed in this long weeding-out process, a time when poor students are forced to come to grips with their failures.

"There are bound to be repercussions," said Kanazawa, "when so many students are treated like dunces. They rebel and torment their teachers or peers. Their sense of frustration usually finds its outlet in attacking weak targets."

Newspapers abound with almost daily reports on the latest outbreaks of what has been dubbed an epidemic of "blackboard jungle violence." In one incident, a young, pregnant teacher was knocked to the floor, pulled around by her hair and struck in the face after chiding a student for reading a magazine in class.

In another, a group of male students brandishing bamboo swords smashed dozens of windows at their junior high school in southern Japan before turning on teachers who had stepped in to end their rampage. They reportedly had been angered by their failure to gain admission to senior high school.

These incidents came to light last month following the arrest of a 38-year-old English instructor after he allegedly stabbed a student attacker with a fruit knife. The teacher, a victim of the 1945 atomic bombing of Hiroshima and in frail health, had been the target of repeated assaults by a gang of campus toughs, press reports said.

In profile, authorities here say, many of the young offenders come from broken homes on the low side of the country's economic scale where discipline is weak. And although parental child abuse is rare in Japan, they have frequently been subject to schoolyard beatings by their peers or corporal punishment by their teachers.

In an older Japan, said sociologist Yuko Akatsuka, "You always had bullies and hangers-on who operated on their own 'neighborhood' turf. When teachers or adults were present the bully couldn't exert his influence. Now, all that has changed, and the classroom itself has become a major venue for violence."

Akatsuka has been appointed to a blue-ribbon panel of scholars and educators set up by the government last month to seek ways of coping with the country's juvenile troubles. In a sign of growing concern among Japan's top leadership, Prime Minister Yasuhiro Nakasone has promised to give the matter "immediate attention" by establishing a Cabinet-level task force to review Japan's educational system.

The spree of adolescent violence in Yokohama has helped prompt such action because, to many Japanese, it paints a possible future scenario for terror at the hands of callous, thrill-seeking young rebels, something undreamed of here until only recently. In Japan, with its highly touted social stability and rock-bottom crime rates, such shenanigans almost always have been associated with social ills thought to afflict the United States and Europe.

According to Hiroshi Okeya, a criminal investigator for the Yokohama police, the youths arrested in the dragnet last month were, in effect, spiritual "dropouts" from a system in which they saw little hope of advancement. They normally met at their hangout, a local video game arcade, and had histories of extorting money from their classmates, shoplifting and other minor offenses.

"They saw their actions as a logical extension--there was the feeling it was all a game," he said.

What has stunned the Japanese is the youngsters' apparent lack of remorse, something individuals here traditionally express when they run afoul of the law. One of the youths told police, "When we beat up the bums I suddenly felt liberated." Another was quoted as saying, "We were cutting classes and going to the game center everyday. That got to be a drag, so we thought it would be a kick to beat up those guys in a sudden attack."

On a recent sunny afternoon in Yamashita Park, where a number of the beatings took place, a junior high school student with slicked-back hair and a 1950s-style outfit reminiscent of the U.S. film "American Graffiti" was asked about the incidents. Gyrating to the rock-and-roll tunes blaring from his stereo tape cassette, he said he preferred to work out his frustrations through dancing.

"But I can't say I wouldn't do anything like that," he added. "No one would dare do it by themselves. When your friends start something, though, it's hard to say 'no' because you might be booted out of the group."