Some people will find it grimly ironic that the first gold medal of the Moscow Olympics, which the United States and about 50 other nations are boycotting because of the Soviet military presence in Afghanistan, was won by a Red Army marksman in pistol shooting.

Alexander Melentev, 26, who 11 years ago was told by an instructor at his school that he'd never learn how to shoot, won the free pistol event. The competition places marksmen at a distance of 50 meters, firing at a target only 500 milimeters in diameter.

Each contestant gets 60 shots, which must be fired in six 10-shot series within a 2 1/2-hour time limit. Melentev scored 581 out of a possible 600, an Olympic and world record.

The Soviets hosts and their Eastern Bloc comrades were right on target in their aim to monopolize gold medals and world records in the absence of atheletes from the United States, West Germany, Canada, Japan, China and other boycotting countries.

Medals were presented for five events in four sports on the opening day of competition. The Soviets won four of them -- including their first gold medal ever in swimming (Sergei Fesenko in the 200-meter butterfly). East Germany took the other, in women's swimming, which is expected to dominate.

The East German 4x100 meter medley relay team had no competition to speak of and still pared more than a second from the world record it set in the 1976 Olympics in Montreal.

Rica Reinisch swam the first lap for East Germany in 1:01.51, equaling her countrywoman Ulrike Richter's world record for the 100-meter backstroke and giving a lead of 2.29 seconds to Ute Geweniger. By the time Andrea Pollack and anchor Caren Metschuck hit the water, the race was a foregone concusion, but they wanted a record.

They got it impressively, clocking 4:06.67 to erase their Olympic and world mark of 4:07.95. Great Britain finished second, more than five seconds behind at 4:12.24.

Earlier, East Germany's Barbara Krause set an Olympic and world record of 54.98 in the third of four preliminary heats in the 100-meter freestyle.

This was the first time any woman has bettered 55 seconds in this event. As her time flashed on the scoreboard at the new Olympic swimming complex on Prospekt Mira (Peace Avenue), it was greeted by chants and applause long before the announcement was made in Russian, French and English.

Krause, in Lane 4, looked rather impassive and nonchalant. But, in Lane 3, Carina Ljungdahl of Sweden, who finished third in the eight-woman heat, flipped over, glanced to her right and rolled her eyes, as if to say: "Are you kidding? That fast in a preliminary?

Krause's performance was early evidence that the East Germans, who swamped everybody at Montreal but were in turn surprised by the resurgent Amercian women at the world championships in Berlin two years later, consider the clock their primary rival now that the U.S. team is not in Moscow.

The boycott, and the effects it will have on people's preceptions of victories achieved in these somewhat less than Olympian Games, were unavoidable subjects of discussion as competition began in 16 sports.

The party line was voiced after his race by Fesenko. He got off to a bad start but finished strongly and was accompanied by tumultuous applause the last 20 meters even though he failed to break the world record of Amercian Mike Bruner. Said the U.S.S.R's first male Olympic swimming champion: "No matter who was in the race, a gold medal is still gold."

Nevertheless, the Soviets dominated opening competitions in the first Olympics in a Communist country with performances that would have earned favorable reviews even if the boycott had not depleted the field in many events.

Their self-satisfying day began on the Dynamo shooting range in Mytishchi, a town northwest of Moscow.

Here Melentev was four points better than the previous world record in free pistol held jointly by Moritz Minder of Switzerland and Paavo Palokangas of Finland.

"When I was in school I came bottom in shooting. I went to the shooting club the next day to practice," said the burly Melentev, whose 581 beat silver medalist Harald Vollmar (568) by 13 points.

The Soviet Union scored a second victory in the 100-kilometer team cycling race over the Moscow-to-Minsk highway. The Soviet cyclists were fastest at all checkpoints and finished in 2 hours 2 minutes 21.70 seconds, beating the reigning world champion East Germans (2:02.53.19).

The Soviets took another gold in the 52-kilo weight lifting class, won by Kanibek Osmonaliev. He beat Ho Bong Chol and Han Gyong Si, both of North Korea. Han earlier had set a world record in snatch with a lift of 113 kilos.

Perhaps the most satisfying Soviet triumph, though, came in men's swimming, a sport in which the U.S.S.R. is an ascending power. Coach Sergei Vaytsekhoski, 51, sent out a team of coaches six years ago to study training methods in such traditional swimming powerhouses as the United States, Australia, East and West Germany.

Vaytsekhoski was the man chosen to accept the Olympic flag from the Montreal delegation in Saturday's opening ceremonies. Tonight his man, Fresenko, accepted the gold in the 200-meter butterfly from International Olynpic Committee President Lord Killanin.

Americans Mike Bruner, Steve Gregg and Bill Forester swept the gold, silver and bronze in this event in Montreal. Bruner set the Olympic and world record of 1:59.23 that still stands.

Fesenko has never raced against Bruner, a 23-year-old Stanford grad whose 1:59.48 is the best time recorded this year.

He got off to a bad start tonight and was well behind East Germany's veteran Roger Pyttell after one lap, but he took the lead in the second 50 meters and never relinquished it. As it became obvious about 25 meters from the finish that Fresenko would win a roar swelled from the crowd.

The new swimming auditorium has steeply banked yellow stands and is separated by a glass partition from the diving pool.

Fesenko's winning time was 1:59.76. Phil Hubble, one of three Britons in the eight-man final, was second in 2:01.20, Pyttel was third in 2:01.39.

A band played and rhythmic clapping filled the hall as Fresenko led the parade to the victory stand. The Soviet national anthem played as the victor's flag was raised, but to its left was not the Union Jack but rather the IOC flag -- part of the protest by the British Olympic Association. The British and 19 other Western national Olmpic committees are not using their national flags or anthems at the Games.

Hubble also boycotted the post-match news conference, which began after Fresenko and Pyttel waited 15 minutes for him to arrive.

The first question asked Fresenko, 21, was whether the absence of Bruner and other Americans devalued his triumph. The Soviet moderator didn't like the question, and demanded that the inquisitor repeat the name of the publication he represented. It was a sports newspaper in West Germany.

Fresenko sidestepped the question, saying: "I was trying my best to win this competition. The fact that Mr. Carter would not let his athletes take part in the Olympics I think was very incorrect, but even with everyone here I think I would have a chance to win. I wanted to retire after the Olympics, but now I suppose I will have to swim another two years, to meet Bruner and the other Americans."

Later, the man from Dusseldorf repeated his question, asking if he didn't think the medal was tarnished. Frensenko was more direct this time: "I value this medal as much as if the Americans had been here. "Gold, hee said in several different ways, is gold.

Par Arvidson, the Swede who hold the 100-meter world record in the butterfly and swims for the NCAA-champion University of California, finished a disappointing seventh in the qualifying heats and seventh in the race itself in 2:02.61.

"I felt this morning that this wasn't my day. I thought I'd swim better in the finals, but I didn't feel good. My arms were going one way, and my legs another," he said.

"I think it would have been a good race between Fresenko and Bruner," he added. "They swim similar races, they're both strong finishers. It would have been interesting."

As the Soviets and their Communist friends pile up the medals, that could become the theme of this boycott-riddled Olympics.