MOSCOW, JULY 7 -- Six officials of the Chernobyl nuclear power plant went on trial today for criminal negligence in the explosion last year that caused the world's worst nuclear accident.

The defendants charged that design faults in the reactor and not just human error led to the disaster.

The long-awaited trial in Chernobyl, which is expected to last three weeks, is the first legal action taken since the April 1986 tragedy that killed 31 persons and injured 200, turned the surrounding area into a contaminated wasteland and spewed radioactivity across the European continent.

Covered by a limited group of foreign correspondents as well as the Soviet press, the trial has become an important test of Soviet leader Mikhail Gorbachev's policies of glasnost, or openness, and holding public officials more accountable for their actions. The accident has already led to the expulsion of more than 60 members of the Communist Party.

The six nuclear plant officials who appeared in court today displayed no emotions as they firmly rejected most of the criminal accusations facing them, according to the pool report by the foreign correspondents allowed to attend.

Viktor Bryukhanov stood first, identifying himself as the former director of the damaged plant, 12 miles from Chernobyl, a small town in the Ukraine. The five other defendants, including former chief engineer Nikolai Fomin, huddled in the center of the makeshift courtroom in the local House of Culture, guarded by armed security forces, as they awaited their turn before the presiding magistrate.

The judge said the court was convening to hear "the violations of rules of technical security." In a dispatch on the trial, the official news agency Tass said that the six defendants were being tried for "a breach of safety regulations at explosion-prone plants."

In addition, the findings of the investigatory commission accused the plant's employes for failing to follow regulations for plant safety, of weak training and of giving shoddy instructions to workers.

The judge, Raimond Brize, the deputy chairman of the Supreme Court, sat in a black high-backed chair emblazoned with a hammer and sickle. Turning to the accused, he asked direct, simple questions: names, work history at the plant.

Each of the defendants seemed to grapple with the answers in his own way. Bryukhanov and some others responded in soft voices, barely audible across the courtroom, while Anatoly Dyatlov, a former assistant to the chief engineer, clinched his fists on the table and closed his eyes when answering.

While accepting professional responsibility, the defendants believe they are not guilty of most criminal charges, according to a court report read at the beginning of today's proceedings.

Bryukhanov was not at the plant when it exploded, arriving only at 2 a.m., according to the court report. Fomin, who came in the wee hours of the morning after the incident occurred, blames the design of the plant and the technical shortcomings for the accident, the report said, and Dyatlov blames it all on a construction fault.

"With so many deaths," he said in court today, however, "I cannot say I am completely innocent."

As the senior official at the site when the experiment started, Dyatlov is considered most vulnerable. He is accused of sending four men to inspect the reactor without informing them of any danger.

Boris Rogozhin, shift chief of the reactor at the time of the blast, said he carried out his duties correctly and did not know of the details of the experiment, the report said.

The other two on trial are Alexander Kovalenko chief of reactor No. 4, where the explosion occurred, and Yuri Laushkin, senior engineer of the industrial technical department.

While Bryukhanov, Fomin and Dyatlov were named defendants earlier and have been in custody, the others are still working at the plant and were only recently publicly identified as accused.

By trying only officials who were based at the plant, the Soviet Union is also implicitly blaming them for not informing regional or national officials until the incident was out of control. The experiment that led to the explosion was conducted without the approval of higher authorities, the investigatory report read in court today said.

The report said that compounding the blame for last April's explosion was the fact that precarious safety circumstances had prevailed at the plant previously, including a near-mishap in 1985.

The courtroom was packed with employes at the Chernobyl plant, relatives of victims, and a handful of journalists -- 150 people in all.

Even today, as people arrived in the courtroom, technicians in white uniforms checked their clothing for radiation. The 18-mile zone surrounding the damaged plant has been closed off from the public since the accident.

An official said that radiation levels in Chernobyl are now one-tenth of a milliroentgen -- four times higher than the area background level before the accident but within international safety limits.

Chernobyl, once a town of 12,000, has become a communications, transport and service center for the continuing clean-up operation at the nuclear facility and surrounding area.