For a while, "A Night in Old Vienna" was almost a victim of its own success last night in the Kennedy Center. As patrons filed out of the Concert Hall and began standing around the Grand Foyer waiting for the dance music to begin, the crowd quickly became too tightly packed for dancing. Nobody seemed eager to go home, and the Concert Hall had not only been sold out but had several hundred people seated on the stage and more standing in back.

"Move back, please," said violinist Alexander Shcneider, sounding more like a bus driver than one of the world's outstanding chamber musicians. "There's plenty of room in the back."

And indeed there was -- packed tightly in front of the Concert Hall, the crowd was thinned out enough at the Opera House to allow ample space for waltzes and polkas on the rug, and even for an occasional gallop -- which can be hazardous when there are pedestrians nearby. The opening number was none of the above -- it was the "Radetzky March" exquisitely played by a few strings without the brass and drums in which it is usually garbed, and people discovered right away that they could polka to it.

Far down the hall, at the Eisenhower Theater, two couples danced in solitude and half-light -- and kept on dancing even when the music stopped. It was not that they couldn't hear the music, but that they didn't need it. The sound system carried it clearly the whole, vast length of the Grand Foyer, and at that distance it sounded pleasantly like music from an adjoining room.

As the crowd thinned slowly, dancing became possible the whole length of the hall, and the crowd divided itself naturally into dancers, who occupied the center of the floor, and spectators content to watch the dancing and enjoy the music. Moving gracefully across the deep red rugs, under the long line of chandeliers, the crowd did manage to have a sort of Viennese air -- aided considerably by the music -- although it was dressed like a Washington party crowd. Or even several party crowds, since the dress ranged from blue jeans to black tie, with a few even dressed in white tie and one man dressed like an Arab sheik, leading a conga line.

The dance band was drawn from the group that had opened the evening with a chamber music program in the Concert Hall. Billed simply as "Alexander Schneider and Friends," they included some of the most distinguished names in American chamber music: Besides Schneider, violinists Felix Galimir and Isidor Cohen, violist Walter Trampler, cellist Leslie Parnas, bassit Julius Levine and pianists Claude Frank and Lilian Kallir.

The formal program offered three distinct styles of Viennese music -- each in an exquisitely idiomatic performance. Pianists Frank and Kallir played the Mozart Sonata in D for two pianos; Schneider was joined by Trampler, Parnas, Levine and Frank in the "Trout" Quintet of Schubert, and a string ensemble played two Landler of Josef Lanner.

A Landler is an envolutionary predecessor of the waltz -- something like a country dance and something like a minuet, and even more Viennese in flavor than the waltz. There has been French, American, even Russian waltzes -- but the Landler belongs totally to Vienna. Last night's performances were precisely, delightfully in the spirit of the music, spiced with little hesitations and tiny touches of portamento, the melodies lilting and the phrasing like that of a singing voice. These two brief pieces were humble but they were a highlight of one of the year's finest concerts.

The two pianos in the Mozart were played with an almost telepathic rapport, knitting their phrases in a dialogue without disagreement.

The highlight of the program was, of course, the "Trout," played with the same specially Viennese flavor heard in the Lanner pieces. The audience applauded after each of the five movements -- a sign, perhaps, that most of them were not the usual Washington chamber music crowd. But this wealth of applause was richly deserved. The ensemble played, as billed, like a group of friends, enjoying the music and one another, and it was a delight and a privilege to be allowed to join them.