A crowd at the Capitol last night estimated at 45,000 sang "Happy Birthday" as a surprise to the 50-year-old National Symphony. It was the orchestra's free annual Labor Day concert, and quite a success for both the musicians and for guest conductor Henry Mancini.

Before the concert, the Oscar-winning maestro was as excited as any other tourist, having his picture taken in front of the Capitol and talking to passersby. His has been a rich legacy; certainly the Hollywood sound in the '60s meant the Mancini sound. And after his score for the movie "10" this year, his popularity reached a new generation.

He had suffered the ignominy of ignominies: A New Wage group, the B-52's, came out with a hit single sounding dangerously like a Mancini tune. The theme in question came from Peter Gunn, he said before the performance.It also has been featured in the film "The Blues Brothers."

"But," he said , chomping on his cigar, [at least] they paid for it."

The current legal dispute may not win Mancini friends of fans of the B-52's, but he certainly did not lack friends last night. In a mostly Mancini program, his 20 Grammies and three Osars were easily explained. The centerpiece of the first half of the evening was the theme from Staley Donen's "Two for the Road." That quintessential film of the '60s occasioned in Mancini bittersweet jazz melodies which have been inmitated to this day, and the NSO strings brought out most of the opuilent melancholy of the film and its music.

Although he is seldom given enough credit, Mancini's contribution to the films of Donen and Blake Edwards was no less important than that of Rota to Fellini, or of Herrmann to Hitchcock. The crowd went wild over his souvenirs from Edwards movies, from the various "Pink Panthers" to the current "10," from which he also included Ravel's "Bolero" in what must be the symphony's most gloriously vulgar performance to date. The echoes of Legrand in Mancini's own score to "Who is Killing the Great Chefs of Europe?" proved that even his less successful efforts work in concert, although a sample from a collaboration with Doc Severinsen sounded as one would expect.

Everyone joined in to clap along in a tribute to the late Arthur Fiedler. There was also a "Battlestar Galactica" selection that was echt middle-age disco; and an unspeakably awful arrangement by Mancini of Beethoven's "Moonlight" sonata. But no complaints were heard, and even the trashy chinoiseries of the "Revenge of the Pink Panther" were cheered. And why not? The National Symphony has been playing beautifully this summer, and with Mancini they gave us a wonderful ending to a long, hot summer.