The special Boston Pops magic began to happen near the end of last night's concert at the Lincoln Memorial. The orchestra struck up "The Stars and Stripes Forever," and after a few notes 50,000 people were on their feet, dancing and clapping in rhythm.

It was the kind of music that has been known and loved in Pops concerts for a century. It was also the climax of Washington's long-lost July Fourth concert, happening 10 days late.

The great Sousa march evoked the sense of fun, the spontaneity and audience involvement that are hallmarks of Pops concerts. And at its climax there was another Pops specialty: surprise. Hundreds of red, white and blue balloons were released, and the crowd uttered a delighted "ooh" as they began to drift, briskly and in remarkably good formation, toward the State Department.

The audience, though smaller than the 200,000 predicted, was still spread on both sides of the Reflecting Pool all the way to its end. A few climbed trees to get a better view of the orchestra. Others went wading in the pool but left after being warned facetiously that the Park Service had infested it with piranhas. Cars filled every legal space available for at least a mile and quite a few illegal spaces on the access ramps to nearby freeways and bridges. A few were parked as far away as Dupont Circle.

The sound system included loudspeakers fairly close to the Lincoln Memorial and others about one-third of the way down the Reflecting Pool. They gave a sound that was generally clear and musical if not brilliant, but at one spot the audience heard sound out of phase from three sources, so that "America" sounded like "America-erica-erica."

The program's strongest material was concentrated at its end. "Stars and Stripes" was preceded by the "St. Louis Blues" March and "The Battle Hymn of the Republic" -- solid favorites beautifully played by an orchestra that ranks with the world's best. A 350-voice chorus, drawn from most of the choral ensembles in Washington, gave a stirring performance of the "Battle Hymn." Then, as an encore (not shown in the concert's delayed telecast), the orchestra played a segment of the "E.T." sound track -- the strongest of several pieces by composer John Williams that were featured by conductor John Williams.

Earlier, the music was hardly above criticism, but it was all American, fairly simple, highly optimistic and almost aggressively inspirational -- the kind of music we like to hear outdoors in July with a beloved national monument in the background. Williams did use the occasion to promote his own music, but he did not make the mistake that had been made July 4 in an all-Bernstein concert conducted by Leonard Bernstein. At no point did the audience have to wonder what was going on or why. Williams' music is not really better than Bernstein's, but it is more appropriate for an audience that is picnicking on the grass.

Williams is a technically competent but not very imaginative composer, at his best when his invention is stimulated by the images he must accompany in a movie sound track. His music is not out of place on Pops programs, but yesterday he played rather a lot for a concert that lasted only an hour.

Among the Williams pieces performed, the "Olympic Fanfare and Theme" and the "Cowboys" Overture sounded a bit like watered-down Copland, but they are pleasant, undemanding music and were well applauded. "America: The Dream Goes On" is a short cantata for soloist, chorus and orchestra that delivers exactly the kind of self-congratulatory platitudes its title promises. The music is pure formula, of a kind that stirred American souls during World War II -- and perhaps we should be worried that it is stirring souls again in the era of Ronald Reagan and the Shiites. In any case, it received more than polite applause.

Some of the applause, of course, was for soloist John Denver, a voice and personality ideally suited to the music. The applause was stronger for "Take Me Home, Country Roads," which Denver sang as an encore, and the audience joined him in the choruses, unbidden, with more power and enthusiasm than it mustered for a sing-along medley that included "We Are the World," "Reach Out and Touch Somebody's Hand," "What the World Needs Now" and "Let There Be Peace on Earth." Perhaps they sang along better because they knew the words better. But, in fact, "Country Roads" was one of the best numbers on the program -- certainly better than "America: The Dream Goes On" and no less patriotic. Otherwise, until the blockbuster end, Williams managed to keep the program relatively free of music that was better than his own.

If this was a conscious choice, it threatens a serious weakening of the Pops' programming, which has traditionally included some material calculated to stretch, gently, the audience's musical tastes. It is not necessary to do this on a ceremonial occasion such as a 100th-anniversary concert aimed primarily at a television audience.

But it would be an artistic, if not an economic, mistake for the Pops to become exclusively a celebrity organization. That kind of status tends ultimately to absorb all available energy, to blunt creativity and to lock the celebrity into routine repetition of a few familiar gimmicks. In its own curious way, the Pops has been a creative force in America's musical life for 100 years. It would be a pity if it settled for less in its second century.