MOSCOW, SEPT. 2 -- West German pilot Mathias Rust said today that he made his daredevil flight across Russia to Red Square in May to meet with Soviet leader Mikhail Gorbachev and press for world peace. But, he added, that turned out to be "the biggest mistake of my life."

On the opening day of his trial before the Soviet Supreme Court, Rust pleaded guilty to the charges of violating international flight rules and illegally crossing the Soviet border. He first told the judge that he was also guilty of "hooliganism," but later reversed himself, saying he had taken special precautions to avoid creating public disorder.

In his first appearance outside a KGB prison in three months, the tall, clean-cut 19-year-old told of encountering a Soviet plane en route to Moscow, and of ducking down to low altitude not to evade radar but to melt ice forming on his wingtips.

Rust looked tired and drawn. He delivered an 80-minute statement to the court with ease and without notes, however, couching his description of the audacious journey from Finland in terms of apology.

"I regret what happened," Rust said at one point. "My chief aim was to make an impact on world opinion. I saw no other possibility to achieve my goal." He later confessed, "It's the greatest mistake I've made in my life." Rust's mother echoed the apology in her testimony.

Rust's saga began May 28, when he flew a small Cessna from Helsinki, the Finnish capital, across the Soviet border and five hours later landed at the edge of Red Square, one of the most heavily guarded spots in the world.

The violation of Soviet airspace led to the ouster of defense minister Sergei Sokolov and a shake-up of military officials that continues.

On the militarily sensitive subject of the Soviet plane that saw him, Rust said "there was visual contact" but the craft, after circling, flew off without interfering.

While most of the flight was at 2,000 feet, said Rust, he dropped down to 1,000 feet occasionally to catch warmer air for de-icing.

Rust, who had time to chat and sign autographs in Red Square before stunned militia arrested him, has remained until today in Moscow's Lefortovo prison, a Czarist-era institution now used by the KGB secret police. The opening of the three-day trial gave Rust his first chance to tell his own version of his story to the public.

Flanked by two military officers, Rust stood in the wood-paneled courtroom packed with Soviet officials, West German diplomats and his parents and younger brother.

The Foreign Ministry invited 25 western journalists and several western diplomats to cover the trial. This report is based on pool reports from sources present in the courtroom.

Rust first listened to Judge Robert Tikhomirov read a 50-minute statement outlining the three charges against him, then took the stand. His German was translated simultaneously into Russian.

"The guilt is clear to me," Rust said in German.

"On all three points?" the judge asked.

"Yes," Rust said.

Rust said he knew Moscow only from a map and piloted the plane toward the center of the city until he spotted the massive Rossiya Hotel across from the Kremlin. He said he chose a spot near the Kremlin's Spassky Tower, next to St. Basil's Cathedral, the colorful church in front of the Kremlin. There were only about half a dozen cars there and no pedestrians, he explained, allowing him to touch down without causing panic.

Under questioning from his Soviet attorney, Vsevolod Yakovlev, Rust told the court that he was inspired to make the mission to Moscow after his disappointment with the summit between Gorbachev and President Reagan in Reykjavik last October, a meeting that broke down in a dispute over Reagan's Strategic Defense Initiative.

"Last autumn in Reykjavik," Rust said, "the U.S. president and the Soviet general secretary met. I wanted to see what would come of the summit. I found it a shame that there were no positive results. Those past events affected me a lot.

"My goal was to make contact with the leaders of this country," he said later. "I had hoped that this would give the possibility of meeting with the leader, Gorbachev, and to explain to him my ideas."

Monika Rust, the defendant's mother, took the stand to corroborate that Mathias considered Gorbachev the best hope for peace. "He is a young man of a noble heart," she said, "but he is a young man lacking in experience."

At one point, Judge Tikhomirov questioned Rust's commitment to peace, asking whether he had participated in any demonstrations in West Germany.

Rust said that he had not.

Tikhomorov later admonished Rust, saying that if he wanted to press for peace he should have landed in West German Chancellor Helmut Kohl's yard instead of at the Kremlin.

"I was seeking the source of peace," Rust replied, "and the source of peace is not in Bonn but in Moscow."

If Rust is found guilty he could be sentenced to up to 10 years in prison. An earlier charge of spying has been dropped. During three months of interrogation, Rust apparently was able to convince questioners that he was not acting on orders from a foreign government. He told the court he had planned the flight alone, without considering the consequences.

Rust arrived at the courthouse in a van protected by curtains and was hustled into the courtroom without fanfare. His parents and 15-year-old brother, who live in Hamburg, were led into the trial by a representative of the Hamburg-based magazine Stern, which has bought the rights to Rust's story.

The family sat somberly near the front of the courtroom throughout most of the day's hearings, with the mother clearly fighting tears all day.

Rust repeated his appeal for mercy in a consistently sorrowful tone. "I threatened the lives of people," he said at one point. "That's my opinion today. I will never repeat it," he said of his flight.