USA Today published this graphic illustrating how the park service estimates crowds and showing the crowd sizes for various inaugural ceremonies:
Based on images like this:
it seems the crowd stretched from the captiol almost to the Washington Monument. I saw a better image showing that earlier, but I can't find it now.
So, if the crowd today was as dense as a typical inauguration, then it seems like there were probably as many people as there were for LBJs inauguration: 1.2 mil. Even if the crowd was 1/4 as dense, that would still mean there were 300,000 people there. I thought anything over 100,000 would be a significant statement. I didn't think we could get 1 million.
All the news agencies, including fox, kept saying 'tens of thousands'. Yeah right. If just the section in front of the capitol holds 240,000 (densely packed), then there were EASILY 100k.
It would be interesting to compare aerial photos of the March on Washington with Obama's inauguration and aerial photos of other events of known size.
I think the best estimates, just based on the photos I have seen and the USA today graphic, would place the likely number at 750k - 1.25 mil.