Since crowd estimates are still being bandied about, I’ll just offer this e-mail:

I’d guess there were somewhere between 150,000 and 200,000 total. This is based on the relative spacing between people in your photos–which correspond to somewhere between pedestrian “level of service” B and C, and allowing for a lower flow rate of 20-25 persons per minute per meter of street width, during the peak hour of the 9-12 march, for Pennsylvania Avenue (which is 100 feet, 30.5 meters. Allowing for curbs, I’d say “usable width” was 28 meters. Thus you get a peak flow of around 40,000-45,000 persons, and less during the “shoulder” hours (before and after) over the 3-4 hours the march went on.
I’d say the definitive figure will be the Metrorail ridership numbers for Saturday, the 12th. The Obama inauguration saw around 1.1 million Metrorail trips, compared to a normal winter Saturday of around 150,000-150,000 (source: Metro website). Assuming this group rides Metro at the rates of other political demonstrators, Fourth of July celebration riders, and run of the mill tourists, I’d guess you’ll get around 40%+/- arrived/departed by subway. Therefore, if there were 150,000 9-12′ers PLUS the normal tourist loads, there should be around 420,000-430,000 Metrorail riders reported for the 12th–compared to about 300,000 typical riders on a Summer/Early Fall Saturday in D.C.,; if there were 250,000 9-12′rs, Metrorail ridership would have been around 500,000 that day.
I wish the NPS would survey not only tourists but political demonstrators so we could get reasonably accurate estimates of crowds. This could be easily estimated based on several surveys of knowing how tourists/demonstrators arrive on the Mall.
That’s my two cents worth, from an Obama-supporting transportation planner…

That seems very reasonable to me. I realize there are those who want to either downplay or overplay the figures, but 150 to 200k — though I wouldn’t be surprised if it was slightly more — seems to be a lot for a variety or reasons. 1) Those on the right side of the political spectrum don’t really have much of a tradition of protests, so the fact this many turned out is remarkable in and of itself. 2) This is an admittedly unscientific impression, but I talked to dozens of people and almost no one I spoke to was from less than a state or two away. Many drove hours and hours to be there or came from the west coast — I even spoke to a woman who came from Alaska. The effort involved in getting there speaks to the intensity of the opposition. 3) Further, thousands more went to local tea parties held the same day closer to their homes. 4) The character of the protesters was very different than the typical protest. A lot of “stunningly normal” citizens as opposed to anarchists and liberal interest groups. Politicians are a lot more concerned about small-business owners, professionals, and seniors taking to the streets than a bunch of troubled kids in black bandanas.