Crowd estimates from Women’s Marches on Saturday now tally over 4 million and political scientists think we may have just witnessed the largest day of demonstrations in American history.

According to data collected by Erica Chenoweth at the University of Denver and Jeremy Pressman at the University of Connecticut, marches held in more than 600 US cities were attended by at least 4.2 million people.

“Even using a conservative estimate, it was the single largest day for a demonstration in the US,” Chenoweth, an expert on political protests and civil resistance, told us.

Every state in America hosted a Women’s March, as you can see in the map above. The events ranged from tiny gatherings in small town squares to throngs of more than 500,000 people clogging streets in cities like Washington, DC, and Los Angeles.

Pressman said he started to track crowd numbers from organizers, local media outlets, and citizens who emailed or tweeted with links and reports early Saturday morning.

Early stages of spreadsheet on counting marchers in US/world today. Working on getting linked sources & more numbershttps://t.co/eILpl0BkFz — Jeremy Pressman (@djpressman) January 21, 2017

Chenoweth offered to help him, after being struck by how many people she saw at the march in her hometown of Denver.

“It was much bigger than I expected, and by the time I got home I was really curious if that was happening nationwide or not,” said Chenoweth.

Chenoweth said she was stunned by how many communities across America held sister marches to the main event in Washington, DC. She and Pressman received data from places like Stanley, Idaho (population of 63), where nearly half the town — 30 people — came out to protest, including resident celebrity Carole King.

Half the town of Stanley, Idaho (pop. 63) came out in a snowstorm to march in peace & solidarity w/ men, women, & children on 7 continents. pic.twitter.com/tLPCognu2t — Carole King (@Carole_King) January 22, 2017

The turnout at events outside the US was significant, too. Chenoweth and Pressman have recorded over 200 international Women’s Marches with an estimated attendance of more than 307,000.

Chenoweth cautioned me that while 4.2 million Americans protesting on Saturday may be the largest turnout in US history in absolute terms, she wasn’t sure if it was the largest protest proportionally speaking. For instance, she said, it’s possible that protests in cities around the US against the Iraq War in 2003 may have drawn as many people or more relative to the population at that time.

Chenoweth says she hopes that civic organizers will be more involved with gathering crowd data in real time to help researchers who study social movements.

“For people who organize these kinds of activity, there is something that can be learned in terms of techniques of using [satellite images or aerial photos] to estimate crowd density,” said Chenoweth. “It might be a good time to think about how we democratize that knowledge.”

Update, January 23, 3:06 pm: We have updated the map to reflect the 10,000 people who attended the march in Helena, Montana.

Update, January 31, 10:00 am: We have updated the map to reflect the researchers’ final, “best guess” crowd estimates of 4.2 million.