[Rain Man]

There were some similar types of things after World War I when ex-military people went to Washington to try to get their bonuses or pensions or something early.

I think in the Depression there was at least one big march to Washington related to the Dust Bowl or some such thing.

In the 1970s there was some sort of tractor convergence on DC for some farm issue that may qualify.

I think various unions march in Paris for all sorts of reasons like only getting seven weeks of vacation instead of eight.

It seems like you could get some pictures from various despot falls like Iraq, though those may technically be mobs instead of marches. The Iran election protests and Tiananmen Square might work.

While not a march, a potentially good alternative would be the destruction of the Berlin Wall. I bet that's well documented, and from what I recall, it started small and then kept growing and eventually you had hordes of people knocking it down.