Jazz musicians improvise. Folk musicians do, too. But among classically trained musicians, only organists nowadays learn the empowering art of improvisation.

A virtuosic improviser, Gerre Hancock, organist at St. Thomas Church Fifth Avenue in New York, took the bench of the impressive Schoenstein instrument at St. Paul's Church on K Street Friday. And as strikingly animated as his performances were of Bach, Reger and Franck, the anticipated event of the evening was two pieces he constructed extemporaneously.

In the first, based on the chant tune "O lux beata Trinitas," Hancock created four movements in rich harmonic layers, improvised within the framework of the French baroque style. (Between movements, St. Paul Music Director Jeffrey Smith conducted the children's choir in the original plainsong.) Improvisation is simpler when the parameters are established.

Thus the second work, the free-form "Improvised Symphony for Organ" in four movements--which Hancock composed on the spot, using themes given him just moments before the performance began--was a triumph of creativity. He'd play each theme once through, pause a moment, and then start playing again. The first theme, a hymn by Richard Dirksen, was spare and clear in its sonata-allegro form. The second theme (the urchins' chorus from Act 2 of "La Boheme") was comically out of place on the organ with tolling bells and shimmering atmosphere. Theme number three, the trifling ditty "Hail to the Chief," inspired him to spin weird harmonies and unexpected cadences. He began the final theme (a moody bass line) as a strict fugue and eventually incorporated the other three themes. That last melody made an impossibly awkward finale, but his success in bringing the darkness to light, and never hitting a false note, was dazzling enough.