Atheists Rally at the National Mall in Washington D.C.

At Washington Post, "Atheists rally on National Mall in show of political force."

And Reason Magazine declaims any organizational affiliations:

Reason.tv headed down to the National Mall for the Reason Rally (no affiliation!) in Washington, DC. The March 24 event was billed as the "largest gathering of the secular movement in world history" and drew a several thousand-strong crowd of damp, enthusiastic unbelievers (and a few protesting believers) to the National Mall.