I’m a devotee of Bach, and I recognized it — it’s the Andante from Trio Sonata #3, BWV 527.
Steen has this to say about the performer:
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From Saturday on the Købmagergade near the Round Tower — He’s a young Russian from St. Petersburg, classically trained form a young age, and plays a certain accordion that is able to imitate the deep notes of the organ.
Just one more reason to love Copenhagen.