BERLIN — The man in the floppy hat and khaki vest was on his knees, pounding two small, brass-plated bricks into the wet sidewalk. One bore the name Arvid Harnack, the other of Mr. Harnack’s wife, Mildred Fish-Harnack — the only American civilian to be executed on the direct order of Adolf Hitler.

On Friday afternoon, more than 50 people huddled under umbrellas for the commemoration ceremony on the front step of Genthinerstrasse 16, the Harnacks’ last address before being arrested by the Nazis for working with the resistance.

The man hammering the stones into place was Gunter Demnig, a German artist who has spent years installing more than 43,000 of these so-called Stolpersteine, or “stumbling blocks,” into the sidewalks of 16 countries across Europe, making them one of the largest decentralized memorials to victims of the Third Reich. Some 5,000 of them are in Berlin alone....