I looked at this shot, presumably taken from the Washington Monument:

I then went to a site I use for mapping and measuring my runs to get an approximate measure of the area on the south side of the Reflecting Pool that appears to be fully occupied by people. That gave me 0.7 miles by 0.41 miles. I then made the heroic assumption that people occupied a similar area on  the north side. If you look at the picture in the top right corner, you can see the crowd  occupying the area to the east of the pond, so I think it’s a fair assumption. The trees may block the view from above but they don’t stop the people on the ground from crowding beneath them. I saw that at the 9.12 rally last year. So, 0.7 miles x 0.41 miles x 2 = 148,665 square meters. A little googling revealed that in a dense crowd, each person occupies 1/4 square meter. That gives me a rough estimate of 594,000 people.

I’d conclude that any estimate of 500,000 or more is fair.

More pictures here.