A throng estimated at 50,000 filled the West Lawn of the Capitol last night and flowed far off, up the double stairway leading to its West Front. They had come to enjoy the balmy evening, perhaps a picnic and the free Memorial Day concert offered by the National Symphony Orchestra. Some music lovers had staked out their spots on the lawn seven hours before the concert.

The NSO has made a seven-year tradition of regaling Washingtonians with free music three times a year: Memorial Day, the Fourth of July and Labor Day. Last night the orchestra's associate conductor, Andrew Litton, was on the podium and TV anchorman David Marash was master of ceremonies.

The concert's first half, to one sitting directly in front of one of the many loudspeakers that flanked the concert platform, sounded like a herd of crazed pachyderms. But the sea of heads bobbing in unison to the metronomic rhythms of Beethoven's Eighth Symphony offered reassurance that the orchestra was in grand form.

When asked during intermission where one could find the best sound, acoustic technician Mike Hoover suggested a seat on those far-off steps of the Capitol. Hoover added, with pardonable pride, that owing to the cunning stacking of woofers and tweeters, the amplification produced four times the sound of a rock 'n' roll group. One could well believe it.

*The high point of the second half was Robert Russell Bennett's brilliantly orchestrated "Symphonic Picture of Porgy and Bess." Compared with Bennett, the other orchestral potpourris on the program, featuring music by Sousa and Cole Porter, was mere journeyman's work. There was also a new piece for bassoon and orchestra by Louis Bellson, who was Duke Ellington's drummer. Called simply "Love," it sounded like a pop ballad, and bassoonist Kenneth Pasmanick played it with the baritonal eloquence of an Andy Williams or Gordon MacRae.