At a signal from the leader, the air near Tokyo's Haneda Airport explodes in a roar of screaming engines. Hundreds of motorcycle headlights bathe the road in blinding brilliance as the youthful parade of speed hurtles headlong into the night on its way to the inevitable confrontation with the police.

Astride powerful bikes that they gun to speeds of 100 miles an hour or more, the members of the motorcycle gangs are Japan's latest wild ones. Their numbers and velocity provide - for a few fleeting hours - the sense of identity and freedom they seek.

Most are high school students or laborers, from 16 to 19 years old, who have failed Japan's fiercely competitive academic system and are doomed to toil in dead-end jobs. Their sleek Hondas, Suzukis and Yamahas are their release.

Every Saturday, in post-midnight forays, they roar through city streets and along shoreline highways behind fluttering banners that proclaim the racing packs as the "Black Emperors," "Zero" or "Specter." To most Japanese they are the bosozoku - the wild tribe.

Police report there are 348 known gangs of bikers and their ranks have grown in recent years to an estimated 20,000 hard-core riders. The speedsters and other friends who sometimes come along for the ride, adding about another 200,000 youths to the motorized whirlwind.

Their credo is disdain for any traffic laws that might slow their noisy progress, and their blatant recklessness sometimes culminates in violence. Takeki Hirotani, director of the traffic division of Japan's national police force, says they are "not only a problem for the police, but also for the educational system, the handling of the younger generation, and the social structure of the nation."

The bosozoku phenomenon is seen as a reflection of a high-pressure educational system that penalizes losers severely. More than 90 per cent of Japan's young people attend high school, but only 38 per cent make it to college. Those who fail the frantic competition of the "examination hell" that determines who can enter a university are sentenced to lifetime jobs with low pay and little prestige.

The leader of the notorious Black Emperors summed it up in a documentary film: "All we have is Saturday night."

The new wave of juvenile delinquency here is largely a middle-class development, bred of affluence rather than poverty. Police say that in more than half the cases they encounter, the lawless rider's bike was purchased or at least partly financed by his parents.

Reiko Ebata, a psychiatrist who counsels bosozoku and their families explains that parents today have more disposable income with which to indulge their offspring. For many youths, alienated from school and frequently lacking the traditional Japanese respect for their elders, motorcycles became their passports to rebellious freedom in the wake of the American film "Easy Rider," Ebata says.

In the highly ordered and rigidly-structured Japanese society, the potent machines provide an outlet for those who, despite material comforts and stable homes, feel bored or hemmed in by school or jobs that they find tedious and unfulfilling, says Ebata. "They want to run along the fine edge of life and death."

Bosozoku, some of whom are articulate and intelligent, acknowledge that they ride to escape. Shoichi Sawao, 25, founder of Tokyo's oldest and biggest group, Zero, with 600 members, has worked 16 hours a day six days a week in his family laundry shop since he was 18. When he bought his bike nine years ago, he would get up before the sun rose and travel for an hour to hurtle along deserted seaside roads.

"There I could forget everything," Sawao recalls, "including the very unpleasant things I had to bear in school or at work and the frustrations I endured in these places."

But the impulse to travel in packs, making daredevil spin turns, shouting obscenities and antagonizing motorists and police, represents more than escapism. It is an open rebellion against authority, according to Ebata. One biker described it as "a desire to crash the strength of our youthful power against something."

Last year there were some 25,000 reported violations of the law by bosozoku, resulting in more than 4,000 arrests. Figures for the first half of this year suggest the bikers have slowed down only slightly.

The worst incident to date occured in May 1976, at the annual festival in Kobe. For the second straight year, the combination of gangs of speedsters and unruly crowds led to rioting in which cars were set afire and store windows smashed. This time, a news photographer was beaten to death.

The incident horrified the nation and led to renewed pressure to curb the bosozoku . A special body was established to coordinate surveillance and the prime minister's office created a commission to study the problem. Thousands of extra police were put on the streets to control the rampaging youths. In Tokyo alone, 1,500 special traffic officers now deploy each Saturday night to try to slow the motor-powered stampede.

The lawlessness extends beyond breaking speed limits and running red lights. Many sniff paint thinner - last year 118 youths died from thinner use - carry weapons and fight with other groups. Some groups have been known to kidnap women from rivals and gang-rape them. Gangsters are also involved with many groups, with bosozoku leaders paying off underworld figures in return for support, protection and weapons.

Despite this behavior, a member generally rejects Japanese middle-class values for only a short period of his life, if at all. Most drop out of the bikers' groups at 20 or 21.

"If people solve their frustrations and find another purpose in life, they will leave the group," says Sawao.

Ebata maintains that while the bosozoku are a spin-off of Japan's intensely competitive society, they do not represent an attempt to break away. "They want the same things their fathers wanted - middle-class lifestyles - but they don't want to work for them," she says. "They look at the long, slow climb up the ladder of success with a sense of 'what difference does it make?'"