The most splendid moment of last night's concert by the National Symphony Orchestra came before the music began. As the players streamed on stage to take their seats before the conductor appeared, the audience broke into a spontaneous tribute to the orchestra on its first appearance here since last month's successful European tour. Finally, concertmaster Miran Kojian signaled the players to rise, and there followed the most prolonged applause that would occur during the evening.

Through much of the concert, the National Symphony sounded like a tired orchestra--not an inexcusable condition for musicians who had played 17 concerts in 21 days as they crisscrossed Europe.

The lassitude of the first half, though, could not be blamed primarily on them. Conductor Rafael Fruhbeck de Burgos chose badly by opening with the lugubrious overture to Mendelssohn's pious oratorio "St. Paul." How one of the most elegant of musical craftsmen could have written this empty exercise remains a mystery.

Then there was the Violin Concerto, introducing a 38-year-old Italian violinist, Uto Ughi. He had a big, lustrous tone, and he chose to use this glowing work as a display piece for that tone. In the process, Mendelssohn's exuberant glow took on a disembodied glaze.

The Bartok Divertimento came next. It is jauntier than most Bartok--hence, the title. But the pain and anguish that haunt most of the composer's work is right beneath the surface, and in the slow movement the pain and anguish take over with compulsive intensity. Fruhbeck's interpretation lacked that extra edge of suffering (in the stabbing high violin trills, for instance).

That improbable and entrancing combination of the circus and the contrapuntal, Weinberger's Polka and Fugue from "Schwanda the Bagpiper" finished the evening with buoyancy, at last. This work demands too much intellectual and technical assurance to belong on pops programs, as last night's version showed.