There’s too much effort in this article to not mention the Promise Keepers event from October, 1997, which was well over 1 million.
Regarding the Promise Keepers event on the mall. Estimates were intentionally downplayed for obvious reasons. I believe more were at PK than at the inauguration...something you won't ever read about. There were side streets lined with men in addition to the mall being filled to capacity. And, after the event- it was noted there was no trash or litter left - as the Promise Keepers are known to leave their events cleaner than when they arrive. I've volunteered at several and have seen it first hand.
I was studying my aerial shots from PK’s 1997 Stand in the Gap, versus Obama’s Inauguration. The crowds were very comparable in size. The PK crowd was denser and much more packed in under the trees on the Mall. The Inaugural crowd extended to the Capitol itself, whereas the PK crowd stopped short of the Capitol. All in all, my honest estimate is that the PK crowd was a bit bigger.
I was at both events and several other contrasts jumped out at me. Of course, PK left the Mall cleaner than when they started and the Obama crowd utterly trashed it. But there was also a shocking lack of energy and enthusiasm from the Obama crowd - there was some occasional chanting and singing, but it never got sustained or intensely loud. The PK guys were singing and shouting with much more duration and enthusiasm.
Disclaimer: I was at the Obama event as a chaperone for my daughter's AP History class, in case anyone is wondering what the heck I was doing there!