In 1967 70,000 came, a giant rally was held at at the Lincoln Memorial. Some 50,000 of them went to the Pentagon, a few hundred tried to rush the stairs and the rest remained outside chanting in order to levitate the Pentagon. The idea being when it levitated and turned oarnge the evil within would be gone. The link below has further details. Flowers were placed in the gun barrels of capital police. Apparently there were arrests for picking flowers in a public park.
Recently a gasline at the home of a congressman's brother was cut when the health care bill was up for debate. A number of other death threats were made as well. Over the last few years people began showing up with guns at rallies. Congressman Gabrielle Giffords of Arizona was shot along with several of her constituents.
This was an event that was both unthinkable and predictable. Rhetoric on Left and Right has moved towards encouraging violence.