Reichsparteitag der Arbeit - 6.–13. September 1937

What used to be a recreational area was changed into a huge propaganda park for party rallies in the years after Hitler getting into power. However, most of the projects were not finished.

Here's a map: Dotted-Pink = planned | red = built

The March-field (German "Märzfeld", März = march, the month, not marching), was supposed to fit 250.000 people.

The Luitpoldarena, which was finalised in 1933 could fit 150.000 people on 84.000 m².

The Zeppelin field and Zeppelin grandstand could fit 350.000 people, out of which 70.000 fit on the stands, looking like this

The Zeppelin field was used in the 1937 rally, so theoretically, 350.000 people were possible. The largest event of the rally probably was the deployment of the "Reichsarbeitsdienst" (~ Reich work service) on Zeppelin field ( image ). The 1937 rally also introduced huge anti air spotlights for an impressive lightshow that were also featured in the "Festliches Nürnberg" propaganda movie. I found numbers for the youth organization rally of the same event, where 45.000 boys and 5.000 girls attended.

For the whole deal, all days combined, an estimate of approx. 1 million participants is used for the party rallies.

Source: Siegfried Zelnhefer: Die Reichsparteitage der NSDAP. Korn & Berg, Nürnberg 1991
Hans Christian Täubrich, Reichsparteitage der NSDAP, 1923-1938, in: Historisches Lexikon Bayerns
Images: Wikipedia / Bundesarchiv

Edit: Bonus fact: At the 1936 rally, 900 of the BDM (Bund Deutscher Mädel, League of German Girls) girls that returned from the rally were pregnant. Apparently, one of the girls named 13 different men as the possible father. Because of this, the BDM was banned from camping in the open at the 1937 rally. (Michael Kater - "Hitler Youth". Harvard University Press, Cambridge, Mass., London 2004)

Also you should know that significant parts of the project were done using concentration camp workers, especially in regards to mining the stones that were used. (KZ Mauthausen especially)