Mozart, who was renowned for his spontaneity and invention at the keyboard, improvised a thirty minute encore at the premiere of his Prague Symphony in 1787, as told here by Volkmar Braunbehrens.
Mozart evidently had a prodigious ability to "compose on the spot"; that is, to improvise at the keyboard. This ability was apparent even in his childhood, as the Benedictine priest Placidus Scharl recalled:
Even in the sixth year of his age he would play the most difficult pieces for the pianoforte, of his own invention. He skimmed the octave which his short little fingers could not span, at fascinating speed and with wonderful accuracy. One had only to give him the first subject which came to mind for a fugue or an invention: he would develop it with strange variations and constantly changing passages as long as one wished; he would improvise fugally on a subject for hours, and this fantasia-playing was his greatest passion.
As a teenager visiting Italy, Mozart gave a concert in Venice (5 March 1771). According to a witness, "An experienced musician gave him a fugue theme, which he worked out for more than an hour with such science, dexterity, harmony, and proper attention to rhythm, that even the greatest connoisseurs were astounded."
Mozart continued to improvise in public as an adult. For instance, the highly successful concert of 1787 in Prague that premiered his "Prague Symphony" concluded with a half-hour improvisation by the composer.
- From Mozart in Vienna: 1781-1791 by Volkmar Braunbehrens, (Harper Perennial, 1991)