Marches across more than 650 U.S. locations on a single day.
Researchers Erica Chenoweth and Jeremy Pressman compiled counts for the 21 January 2017 marches from crowdsourced reports validated against photographs, law enforcement statements, and event pages. Their published estimate is a range — three to five million participants across more than 650 locations — not a single figure. The width of that range is the honest result of the method, and crowd sizes for this day were disputed at the time. The counting effort became the Crowd Counting Consortium, now run jointly by Harvard’s Ash Center and the University of Connecticut.
Organizing was coordinated in part through online tools. No source cited here measures how much the digital coordination contributed to turnout relative to other factors.