I know that my family, and several thousand others, were still standing in line at a single MetroRail station (Vienna station, Orange line) at 10:30 or so when they finally gave up trying to have everyone buy tickets.
The MetroRail system was so overwhelmed, they simply let everyone on for free. We didn't all get into DC until well after 11:30.
That was one station from one line of the MetroRail.
The crowd was huge.
We waited in line to board the metro at Franconia on the blue line from just before 8 until 11AM. We were told about 20,000 had already ridden from this one stop so far. There was no end in sight to the line waiting when we got on the metro. By the time we got to the rally it was after noon. Lots of folks were already leaving and many still arriving the crowd numbers changed all day but it was huge.