IT WAS the quality of the silence that followed the ending of Benjamin Britten's "War Requiem" last Tuesday night that was the unmistakable sign that something very rare, something seldom encountered, had just transpired. From subsequent reports on the succeeding evenings when the National Symphony repeated the "Requiem," the same sense of an extraordinary event was present.

Both nights, as Rostropovich sustained the voices of the Choral Arts society with his lifted arms, prolonging their closing, unaccompained "Amen," the audience, which had been caught up in rapt silence during the last pages of the work, seemed unwilling even to breathe. When the final whisper of sound had faded, and Rostropovich had slowly lowered his arms to his side, there was, throughout the Concert Hall, for a welcome moment, neither sound nor motion.

Then, finally, there came that necessary release of emotion from those who had become more and more deeply involved in the words and music they were hearing. On Tuesday night, it came like a wall of sound suddenly erupting in shouts and applause that signaled the release of the emotional tensions that had been created by overwhelming performance of so great a work. On Wednesday the augience rose to its feet as if lifted by a single, giant hand, to stand there and hail the performers.

This catharsis happens rarely in a lifetime, and, because it depends upon elements that are both tangible and intangible, it cannot be predicted. The Greeks understood the phenomenon when they linked it to the process of purification. As we use the term today, we mean, almost always, a purifying of the emotions and a spiritual renewal achieved through art.

Those things that must be present if this singular experience is to occur are, first of all, a work of the highest caliber. It is inconceivable after, say, an opera of the verismo school such as "Andrea Chenier," or "Tosca," with their melodramatic gorgings. The second essential is artists of the rarest insights. Audiences may be brought to hoarse ravings by sock'em dead tenors or pianists who set new world speed records. But that is not the thing we are talking about.

Tuesday night's remarkable experience was also a reminder that the musical event must take place in surroundings that permit the music to be heard with ideal clarity. Earlier performances of the same "War Requiem" in Lincoln Center and in Washington Cathedral had, for differing reasons, not attained the extra-terrestrial heights that were reached last Tuesday.

Nor is this kind of exaltation possible through the performance of a short work. Any number of the preludes and etudes of Chopin are, like many of the songs of Schubert, Schumann and Hugo Wolf, absolute perfection. But there is an element of cumulative time without which it is not possible to attain this particular artistic elevation.

All of these elements were generously present in this past week's performances of the Britten "Requiem" in the Kennedy Center. First of all, the texts that inspired the composer, the traditional Latin Requiem Mass and the war poems of Wilfred Owen, are superb in form and sound. Owen was a British poet who was killed while fighting France one week before the Armistice was signed in 1918.

The terrible, thrusting poignancy of his denuciation of war is one of the strongest agents in giving to Britten's music what Jacques Maritain says is the unique property of the artist, the power to attack us directly where we are defenseless, in our hearts. To speak personally, there are to me, in the "Requiem," sudden blinding moments of beauty so glorious that their impact is like that of a sword thrust, against whose unexpected edge I have no defense. Such a one is the tenor's song, sung over a whispered chamber ensemble, as he signs, "Move him, move him into the sun -- gently, gently, its touch awoke him once -- if anything might rouse him now, the kind old sun will know."

Another is the quiet drradful dialogue between the one-time enemies (Britten asked for a German baritone and an English tenor for the soloists in the first performance of the "Requiem," to underscore this bitter truth): "I am the enemy you killed, my friend. I knew you in this dark; for so you frowned yesterday through me as you jabbed and killed. I parried, but my hands were loath and cold." Then they sing together, "Let us sleep now," as the music moves toward its close.

But the final glory, in measures of unutterable simplicity but endless beauty, is the final cadence, a sequence of chords that is not is not so unusual, but which is, in Britten's handling of it, one of the moments of supreme musical beauty.

All the required elements were present this week in the "Requiem": music of the highest quality; of a length adequate for the creation of that sense of detachment from the world which is vital; a proper place in which to hear; and interpretive artists imbued with special insights into the deepest meanings of the work itself, and open to that higher inspiration which strikes, like lightning, where and when it will. (As Joan Sutherland said last Monday night about singing: "The rockets and shooting stars do not og off in every performance.")

For those to whom these rare experiences have come, they are moemtns for deepest gratitude. I can remember a "Tristan und Isolde" in which Kirsten Flagstad and Lauritz Melchior had seemed transformed by the familiar music until, as the "Liebestod" ended, the audience sat motionless and silent for moments before it began to offer its inadequate response to what had just happened.

And there was a night in the Library of Congress when Joseph Roisman had led his friends of the Budapest Quartet in a performance of the C Sharp Minor Quartet of Beethoven that went so far beyond what might be called the players' normal excellences that they, like their listeners, sat unmoving when it had ended. Rudolf Serkin, who often says of his playing, "We do the best we can," once played the Opus 111 sonata of Beethoven in such a manner that when he had finished, his hands slowly moved down to his sides and he sat there while the audience slowly began the return to the world of reality from which he had moved them.

Gerard Souzay, singing Schubert's "Winterreise," attained that same glorious region, silencing every desire to applaud for that moment of suspended remembrance.

And there have been performances of Leonard Bernstein's "Mass" whose closing pages have achieved this same unmistakable sense of communal exaltation in an audience. I have even known the phenomenon to occur through the medium of a recorded performance. The first time I heard Wanda Landowska's incomparable recording of Bach's "Goldberg Variations," the perfection with which she moved through its final pages created a time of utter peace, following, as always happens in all of these works, episodes of vast upheaval and turmoil.

This past week it was the coming together of the music and spirit of Benjamin Britten through the mediations of Mstislav Rostropovich, Galina Vishnevskaya, Peter Pears, and John Shirley-Quirk, Paul Callaway, the National Symphony, the Choral Arts Society trained by Norman Scribner, the boys of Washington Cathedral and Richard Dirksen, all in a concert hall whose acoustics were planned by Cyril Harris, that helped to create another of these rarest of musical experiences.