A lightning shutdown of much of Japan's commuter rail network by far-left radicals last month has unnerved authorities here and demonstrated just how easily a few determined people can paralyze a technology-dependent society.

Acting with military precision, helmeted radicals attacked the state-owned Japanese National Railways system at 34 points, most of them in Tokyo, early on Friday, Nov. 29. Trains were idled on 24 lines, affecting 18 million commuters, by official estimate.

The radicals used firebombs and other weapons. But most of the chaos resulted because they systematically sought out and severed electronic cables that run along the tracks and direct the trains' movements.

Trains were running again by evening. But the attacks' stunning success, in a country that prides itself for trains that run on time, shocked the public and led to fears of the country being hostage to anyone with basic technological skills as it moves into the so-called "information age."

"Without using great physical power," says Takuro Suzuki, a commentator who specializes in criminal issues, "it is now possible to threaten the entire mechanism of the city."

In the Diet, Japan's legislature, a piqued Prime Minister Yasuhiro Nakasone denounced the "evil acts" of the saboteurs, some of whom were said to be railroad employes. "I want to establish a system that ensures that this never happens again."

Attention is now turning to the summit of western leaders Nakasone is to host in Tokyo in May. The attacks have led a police committee planning security to focus more attention on city services, rather than just the leaders' physical safety.

"It has signaled more need to secure the communication and transportation systems," said a senior officer at the government's National Police Agency. Noting that overseas communications links will be used heavily during the summit, he said: "If that is attacked, it would a direct shock to the delegations."

The attackers knew what they were doing. They brought heavy-duty shears, lifted the lids off concrete trenches that house the cables and cut them. Many had more than 100 smaller wires inside. According to press reports, they also tried to jam police communications with a special radio transmitter.

It was their most successful high-tech attack, but not their first. In 1978, they cut cables leading to the control tower at New Tokyo International Airport at Narita, stopping takeoffs and landings.

In 1982, they hit the railway system with a similar but smaller cable-chopping action.

Last month's attack came at a time when Nippon Telegraph and Telephone, the national phone company, is rewiring the country with high-capacity optical fiber. Banks and corporations are becoming more dependent on data transfer networks for day-to-day transactions.

Optical fiber is far more efficient than conventional metal lines. "A single fiber the thickness of a hair can handle 5,700 calls," points out Moriyuki Torii, senior researcher at the private-sector Japan Information Processing Development Center. But a single cut will knock out that many, too.

In November last year, an accidental fire in an underground cable conduit in Tokyo knocked out 90,000 telephone lines and served notice on Japan on the communication network's fragility.

Most phones in the city's Setagaya area went dead. Data service on the main computers of major banks such as Mitsubishi and Daiwa was hamstrung. Police and fire emergency lines were cut. It took a week to restore full service.

Japan's radical student movement, flourishing in the late 1960s, has vanished from many campuses today.

Those who remain with it, however, are fervently committed, and authorities fear they will try to make up for their low numbers with know-how.

Their main target in recent years has been the Narita airport, which they call a symbol of government oppression. It opened in 1978 after years of pitched battles between armed demonstrators and police and, despite some of the world's strictest security, continues to be hit periodically by homemade bombs and rockets.

At Narita, students claim to be acting in the interests of farmers whose land was taken over for construction.

The Nov. 29 attacks were ostensibly in support of a railway labor union that has supported the airport fight and also just staged a one-day strike to protest plans to denationalize the rail system. The union denounced the attacks.

Police say the attack was the work of an underground group called Chukaku-ha, or Middle Core Faction, a familiar name in the airport campaign. Last year, it succeeded in burning out much of the headquarters of the ruling Liberal Democratic Party with a firebomb attack.

Japanese police have arrested 38 persons in connection withthe November attacks, including two employes of the railway. Many newspapers and politicians are now calling for tightened security, but liberals counter that a free society is essentially defenseless against such things.