Leonard Bernstein led a birthday bash for his country and a few hundred thousand of his friends last night on the West Lawn of the Capitol. There were also the millions around the country who watched it on television's PBS.

The enormous crowd, jammed under a spotlighted American flag flying over the Capitol, seemed bewitched by the National Symphony's performance of Bernstein's "Symphonic Dances" from his most famous work, "West Side Story." They were hardly less moved by his extraordinarily eloquent set of 12 songs for six singers and orchestra called "Songfest," composed for the Bicentennial.

And they really lost their heads at the coruscating intensity of Bernstein's concluding "Stars and Stripes Forever," which he directed with amazing brio. Estimates of the crowd ranged from 150,000 to 260,000, and most of them appeared to be on their feet for the march. They clapped in cadence until Bernstein slowed near the end to such deliberate but joyful majesty that it was hard for some to keep in rhythm.

Oddly enough, this was the first time that America's most celebrated composer-conductor had directed the annual Fourth of July concert on the Capitol lawn. After last night's success, he should do it every year -- maybe next time with some Copland, in honor of his 85th birthday. Certainly the emphasis should remain on American music.

The "Songfest" is a subtle work, and one had wondered if would work as well outdoors. That uncertainty was removed within about five minutes of its considerable length. Understanding the words of these magnificent American poems that the composer chose from poets ranging in history from Anne Bradstreet of 17th-century New England ("To My Dear and Loving Husband" -- dedicated by Bernstein to Rosalynn Carter) to the swinging jazz of a current poet like Lawrence Ferlinghetti.

Two of the songs are as moving as anything Bernstein has ever produced. One is "To What You Said . . . " a recently discovered masterpiece of Walt Whitman, referring to the great poet's secret homosexuality. The baritone sings a rather tense and slow vocal line, memorably done last night by Chester Ludgin (who was the father in Bernstein's "A Quiet Place" last season). Under the song's vocal line, there is a melody starting in John Martin's solo cello that is as rapturously beautiful as anything in Bernstein's music.

The other song that is heartbreaking in its sadness is Bernstein's version of Edna St. Vincent Millay's sonnet "What Lips My Lips Have Kissed," with the remarkable closing line, "I only know that summer sang in me/ A little while, that in me sings no more." The music was written during the period when Bernstein's wife Felicia was dying.

Bernstein solved the problem of word comprehension by simply having each poem read aloud before being sung, in this case by two of his children, Nina and Alexander, just as they read the lines on the new recording of "West Side Story." Perhaps it's a good idea to do this all the time; at the work's world premiere here at the Kennedy Center in 1977, many of the words were lost.

Earlier this week in an interview, Bernstein said of the premiere, "I did not conduct it very well. I mean this quite seriously. I conduct it much better" now. Last night, there was a breadth in the most lyric moments and a bite in some of the others that exceeded the premiere -- further confirming the work's range and intensity,

In remarks recently on Italian television when Bernstein performed "Songfest" in Rome, Bernstein said the work is "not so much to celebrate the glory of my nation, as that of its artists, specifically its poets . . . taken as a whole "Songfest" is a celebration." He ended the Rome program with the comment, "We are not gods; we are only artists. We struggle with our words and our music, and we praise God for giving us the chance to do so."

Other poets in the "Songfest" include Frank O'Hara, Julia de Burgos, Langston Hughes and June Jordan (their poems are combined in a single piece), Gertrude Stein; e.e. cummings , Conrad Aiken, Gregory Corso and Edgar Allan Poe.

Last night's singers also included Lucille Beer, Clamma Dale (in resplendent voice), Gweneth Beer, Charles Walker and Kurt Ollmann.

The "West Side Story" performance at the concert was fine, but, as Bernstein had feared earlier in the week, it apparently had not gotten quite the kind of minutely detailed rehearsal reserved for "Songfest."

Such priorities were only proper. For last night's painstaking performance removed any doubts that "Songfest" is a major work of the times. It provided a marvelous celebration for the nation's birthday.