After I had discussed the lack of improvisation in classical music with a friend, he showed me this article from WQXR.org (the website of a classical music radio station), which claimed that Mozart often improvised his music:
Improvisation is a nearly obsolete art in classical music these days. But virtuosos used to improvise all the time. Mozart freely improvised on his own tunes, Liszt would strike up an aria from a Wagner opera and embellish it.
Is the claim that Mozart improvised music (presumably in concert) true?