According to University of Connecticut political science professor Jeremy Pressman, between 3.5 million to 4.2 million people marched in the United States alone today (as of 10:30pm EST on 21JAN). These are clearly the largest protests in U.S. history. With solidarity marches across the globe, Women’s March 2017 is without question the largest and most important—and peaceful!—protest march in world history.
Here’s a link to his up-to-date spreadsheet