At its height, my guess is that there were about 500 people at the event near the Reflecting Pool on the west face of the Capitol. (That is an honest and not an inflated estimate.)
This story in
The Washington Post is about Cindy Sheehan but about half way down, it tells the story about the protest. It is pretty accurate, although it gives no estimates on attendance. It also casts the event as an anti-war rally. Concern about the war was an elemental feature of the rally, but it was an unequivocally anti-Bush rally. The story doesn't convey that flavor.
It was very cold, and the winds were gusting. Given the inclement weather, the size and enthusiasm of the rally was particularly impressive. The police presence was very high. There was quite a collection of people there. Young liberals. Older intellectuals. My guess is that people spanned the left-leaning political spectrum. Two of my friends came up from near Charlottesville to participate, and they deemed it the best such event they ever had attended.
The noise that we created was awe-inspiring. I had a whistle that I blew with great enthusiasm. My ears are still ringing this morning from the noise we produced. I hope I didn't damage them in any serious way.