There was an air of improvisation Saturday evening in the Kay Spiritual Life Center of American University, when musicians, political exiles and their friends, and ordinary lovers of music and freedom got together for a concert honoring Solidarity. Arrangements had been made within less than a week, after President Reagan's unexpected announcement of Solidarity Day. The printed program had quite a few typographical errors and the order of works listed was frequently changed.

"Everybody says the typist did it," apologized Edward Lozansky of the Sakharov International Committee, a cosponsor of the event, "but nobody will say who typed it . . . We had been thinking of doing a program like this perhaps in March."

"Music evokes a nostalgia for the future," said composer-conductor Jerzy Sapiewski, who introduced the musical selections. That's what the music did Saturday night.

It was not the usual political rally, not the usual campus meeting of concerned academics, and certainly not the usual concert, though it included elements of all three. There was even a touch of wry poetry in a letter smuggled out of Poland and read to the audience by Frank Turaj, dean of the American University College of Arts and Sciences, which cosponsored the event: "They will crush us when they crush the birds -- when they take away their voices and clip their wings."

The evening's music often reinforced its theme -- not only in the operatic arias by Polish composer Stanislaw Moniuszko, but in the Russian selections sung by mezzo soprano Renata Babak and bass-baritone Andrij Dobriansky and in Italian arias by Verdi. Above all, perhaps the theme came into focus in the funeral march from Chopin's second piano sonata, played superbly by Alan Mandel, though some other passages might have profited from a bit more rehearsal. "This was not Chopin's idea of Poland's future," commented Sapiewski, but it seemed suitable for Poland's present.

Musically, the most thrilling part of the evening was the singing of Babak, a defector from the Bolshoi Opera and outspoken critic of the Soviet government. Her voice is both big and sweet, tremendously powerful and superbly controlled in its upper register, with only a small, piquant touch of the vibrato so often overindulged by Russian singers. Her singing reached its climax, and the evening was crystallized in a pure and colorful expression of what it was all about when she sang "Pace, Pace" from Verdi's "La Forza del Destino" -- an anthem for troubled souls if ever there was one.