Here is an awesome video showing the progressive gathering of the massive crowd at the 9/12 "Taxpayers March on Washington" today. You can see the U.S. Capitol in the distance, where the massive crowd marched to from Freedom Plaza.
The video shows the crowd gradually growing and marching down the street to the Capitol. It is clearly a crowd in the hundreds of thousands if not millions. So far, estimates have ranged from "tens of thousands" to "1.5 - 2 million."