This was the 9/12/2009 March on DC. Photos ranged from the begining of the event (10AM) to the end of the event (4PM). There were a few organized groups that coordinated the effort, but probably half or more of the attendees were just regular people (like me) that came to DC with no affiliation or orgainization. Crowd estimates range from 50,000 to 2,000,000. There was a Park Service helicopter that flew over several times that day, and from reviewing past photos and Park Service estimates, I estimate that the crowd was between 300K and 500K attendees. The Million Man March from 1994 was aiming for 1 million people, and only hit 400,000. I think that this crowd was right around the same size. I have more pictures of solid crowds going for half of a mile down Constitution Ave. and Pennsylvania Ave. Regardless of the exact head-count, this was definitely one of the largest gatherings at the Capitol Building in history... but the mainstream media (not surprisingly) refused to cover it.
The Park Service stated that "other groups" had taken out permits for the center mall area before us, and therefore pushed our groups out onto Constitution and nearby avenues. To me, this would seem like a simple way to make the crowd size appear smaller because "it didn't even fill the mall".