"They were a pretty good audience," said British pop star Elton John, half an hour after finishing a gruelling, 2 1/2-hour concert before a whistling, cheering, capacity audience of Russians.

Still wearing one of the famous floppy caps which hide his hair transplant, baggy Magenta trousers and shiny black plastic boots, John, 32, munched on cold chicken and ham in the midst of diplomats and elegantly clad women who asked for his autograph in the gold and white ballroom of the British embassy. In between chatting with the reception guests, who included poet Yevgeny Yevtushenko and U.S. Ambassador Malcolm Toon, he reflected on the thunderous welcome that greeted his performance.

"Of course, half of them were there just because they got official invitations. But the other half were kids who liked my songs and really gave me a good reception." This was an understatement at best. He had brought his concert to a wild and climatic finish by belting out fortissimo "Back in the U.S.S.R." to whistles of delight and thunderous applause.

For weeks Elton John's eight-concert tour of the Soviet Union has been the main topic of conversation among the smart young set in Moscow. He is probably the biggest name in Western pop music ever to have come here, and he rose to fame overnight in a country that still has not reconciled itself with what it regards as the decadent influence of Western pop culture.

Tickets to the four concerts in Leningrad and the four in Moscow were sold out long before his tour was officially announced. In the 4,000-seat hall in Leningrad where he put on the first show on May 21, only a fraction of the places were sold through the box office. The rest were allocated to those with influence, connections or some claim to represent the official Soviet popular music industry. The same was true in Moscow, where John played to a full house in the concert hall of the giant Rossiya Hotel. Tickets on the black market were reported selling for as much as 150 rubles ( $225) each.

Not a single record of his songs has ever been on sale in the state music shops in the Soviet Union, though officials now say his album "A Single Man" will be released here.

But many young Russians, who have an amazingly detailed knowledge of the latest charts and trends of pop rock, disco and reggae, knew John's songs before he set foot in this country. They have heard them on the Voice of America broadcasts to Russia, the BBC and other foreign broadcasting stations, or made their own recordings from albums left behind by tourists or obtained by travelers abroad. Claps of recognition greeted "Daniel," "Goodbye Yellow Brick Road" and other favorites.

"I came here because I wanted to play Russia," John told Soviet and Western journalists at a press conference a few days ago. He felt he needed a challenge. He did not know what to expect from a Russian audience or how his showmanship would be received. But he was willing to gamble because he felt he would become stale and bored if he stuck to the Britain-Australia-North American circuit.

So, since February 4 - a couple of years after he announced his retirement - he has been on the road touring Europe and Israel, and he has written much more music.

Since December, he and his manager, John Reid, have been conducting tricky negotiations with "Gosconcert," the Soviet state promotion monopoly, to bring John to Russia.

The tour has not brought him any money - he did not come for that, he says. But he reckons it has brought him prestige. "Russia's a prestigious place to play," he told the Soviet journalists, who eagerly jotted down every flattering word the quiet-spoken and articulate John said of his hosts and his tour.

But playing Russia requires plenty of paperwork and preparation. All his songs had to be approved by the censor first. John was delighted that nothing was cut (unlike Cliff Ricard, another British star who came here a few years ago and was prevented from mentioning Israel in one song). He brought an entourage of 27 with him, and the entire set, props and formidable array of lights had to be brought in by truck.

Once here, they experienced the usual mix-ups and frustrations of Moscow hotels - which John did not hesitate to complain about. About the final problem was the weather. Moscow has been having a record heat wave, with temperatures hitting 90 degrees Fahrenheit without the aid of air conditioning.

But John seemed to like it. He said he fell in love with the classical beauty of Leningrad, comparing the city rather bizarrely with Manchester and Birminham. He did not want to come down to Moscow. But of course once here he did the usual sightseeing tour of the Kremlin. He also had himself photographed in his gaudy concert get-up doing a Russian caper - to the amazement of stoic Russian onlookers - and he has been playing his visit for all it is worth to a bevy of journalists both Russian and foreign. John also brought his mother and stepfather along for the tour.

The shows have been professional to the fingertips. Leningrad seemed to go a bit better than Moscow and on the first night there were scenes of torrid frenzy rarely witnessed since the Bolshevik Revolution. Crowds chanted his name, a 100-strong police cordon guarded the stage door, and during the show enthusiats in jeans and long hair threw flowers, ran to the stage and did a lot of uninhibited things the Russians normally do not permit themselves to do.

Moscow was a bit quieter - too many ambassadors, army generals and other unlikely pop fans in the audience - but the stray girl still managed to rush on stage for an autograph and a kiss between numbers.

The opening songs were rather restrained - Russians even said dull. They wanted the clowning, the noise, the beat and the showmanship of a John spectacular. They did not know what to make of the lone, small figure at the grand piano with the single fading spot on him as he sang the haunting "Don't Let The Sun Go Down on Me." But things warmed up. As "Heard It on the Grapevine" (translated in the Russian program in a meaningless literal way) got louder, the lights flashed, John hammered it out on the eletric Yamaha piano, and the whistles and typically Russian rhythmic clapping began. And then in a puff of smoke and a flash of light, Ray Cooper, John's percussionist, burst up from behind the drums like Mephistopheles.

In his pinstripe business suit, crewcrut and thin gold spectacles, Cooper leapt and cavorted about the stage, beating on drums and cymbals and hammering on a huge gong while John kept up a nonstop half-hour of hard rock. This was what the Russians wanted.

Perhaps one reason the Russians dedcided to let John come was his proletarian image. His love of soccer has been widely reported in the Soviet Press. (He manages a small-town English soccer club.) In Moscow, he watched the Red Army team play Minsk, and told the Russians he thought they had some fine players on the field. But when asked for a comment on next year's Olympics, he said he thought they would be chaotic - "They thought they had problems in Montreal." He had no intention of coming anywhere near Moscow during the Olympics, he said.

Predictably, remarks like that were not well received and the Soviet interpreter simply left them out, as were all references to John's tour of Israel. When asked about his opinion of apartheid in sports, John said he opposed all forms of discrimination - whether sexual preference, race or religion. The Russians translated only the references to race and religion - sexual preference is still a taboo subject here, and John is a declared bisexual.

As far as the authorities are concerned, John has been a rather awkward concession. They have found it hard to admit publicly that part of the reason he has done so well is that he is known from foreign radio stations and represents the Western youth-pop-jeans culture, dazzingly attractive to Soviet youth precisely because of its contrast with Communist ideology.

Western correspondents who have reported the whistling and cheering, the scramble to get tickets and the razzmatazz surrounding the tour have been reprimanded for slighting the dignity of Soviet audiences and belittling Soviet culture. Soviet officialdom, which insisted at first on referring to John in the papers as the folk singer Reg Dwight (his real name, used on his passport), can only take an ambivalent attitude to a phenomenon it suspects might set a dangerous trend.

John said he hopes he will pave the way for more big names to appear in Soviet Union, and he made a point of asking the head of Gosconcert whether Paul McCartney, who once said he would play in Red Square, would get a chance to do so. "Nobody plays near the tomb of Mr. Lenin," the Russian replied. (The interpreter translated this as "Red Square is not a place to perform.")

John has already promised to write "a piece of music" to sum up his tour. With Britain about to sell Harrier jets to China, the eccentric pop star has probably done more for the British image in Russia that several manyears of diplomatic niceties. CAPTION: Picture, Elton John after the Moscow concert, by AP.