British rock star Elton John's first concert in the Soviet Union erupted into a frenzy last night, with nearly 4,000 Russians fans dancing on their seats or in the aisles.

Uniformed Leningrad police and other Soviet officials were helpless to calm the screaming, clapping mobs.

The flamboyant rock star, wearing high-heel patent leather boots, a large white cap, a green shirt and blue velvet pants, brought the tumultuous response in the first of eight concerts in the Soviet Union.

Thousands of fans screamed, shouted and whistled John back onto the stage of the October Concert Hall for three encores.

"I'm knocked out," he said. "This has to be my biggest achievement as an artist. These people don't have any records, and yet they reacted like that. I'm at a loss for words."

Between songs, hundreds rushed forward for autographs, which the beaming singer obligingly gave. Girls carried large bouquets of flowers onto the stage at every break in music.

Hundred more fans were left ticketless outside the hall, as the 32-year-old star went through songs familiar to the mainly youthful audience, such as "Daniel" and "Goodbye, Yellow Brick Road." (His music well-known to Russian rock fans, although his records are not officially on sale here. Soviet fans depend on black-market records and tapes and Western radio stations for their music.)

Before the concert, tickets costing a maximum of $9 were changing hands on the black market for up to $150.

At the first strains of the old Beatles hit, "Back in the U.S.S.R.," teen-agers in the audience pushed past official state ushers-a rare show of defiance-and headed for the stage. John was so caught up in the frenzy that he started banging out the song even though it had not been screened by Soviet censors, and even though he didn't know the words. "I just sang 'Back in the U.S.S.R.' over and over again," he said.

John is the first major rock star to appear in the Soviet Union, where even jazz was the officially taboo less than 15 years ago.

"I think as far as rock music goes, they are loosening up a bit here," John said.

The crowd, about half of whom were young people, mostly dressed in jeans, was subdued during the first half of the show. But they came to life when John was joined by percussionist Ray Cooper, who appeared on stage out of a cloud of smoke and a flash of lightning.

At one point, John's electric piano exploded during the song "I Heard It Through the Grapevine," but he scurried comically over to his grand piano and picked up the melody where he left off.

He introduced each song himself in English but rose to the occasion by adding in Russian "Dobry vecher (good evening)" and Spasibo (thank you)."

All the evening's programs, highly prized as souvenirs, were snapped up more than half an hour before the singer appeared.

And long after the concert, hundreds of excited fans gathered outside, shouting "Elton, Elton, Elton," until the singer appeared at the window, waved and tossed flowers.

One hundred Soviet security police were unable to keep the crowd back as it mobbed the singer's departing limousine.

An official with the Soviet agency Goskoncert, which had arranged John's appearance after months of negotiations, said: "I've never seen anything like this. Never."

John said young Russians, were "starved" for Western rock 'n' roll music. "Maybe this is the start," he said.

Though the dates of four John concerts were announced on Leningrad Radio only two weeks ago, young Russians complained that the concert hall was "sold out" by the end of April.

The vast majority of tickets apparently were taken by friends of city officials, high-ranking Communist Party members and their sons and daughters.

Goskoncert sources said, meanwhile, that former Beatle Paul McCartney and his group Wings might tour Soviet Union soon. Also mentioned as a possible candidate for a Soviet tour is American star Bob Dylan. CAPTION: Picture 1, Elton John at Monday's concert, by AP; Picture 2, Elton John at the concert; by Tass