The numbers are in: more than 3 million people protested the day after Trump's inauguration

Leanna Garfield/Business Insider

Protester's at a women's march.

More than 3 million people marched on Saturday, the day after Donald Trump was sworn in as the 45th President of the United States, according to data compiled by University of Connecticut professor Jeremy Pressman and University of Denver professor Erica Chenoweth.

Turnout varied from big cities like Washington, D.C., where around 500,000 marched, to small towns where turnout was in the single digits.

And that's a conservative estimate, according to Pressman.

"3 million is on the low end of our numbers," he told Business Insider. "It could be anywhere from 3.2 million to 4.7 million people."

The protests - collectively dubbed the Women's Marches - were organized to include those who were "committed to equality, diversity, and inclusion and those who understand women's rights as human rights," according to the initiative's official website.

Though there's no way to nail down an exact number of protesters, crowd scientists from Manchester Metropolitan University in Britain estimated - by analyzing video and photographs- that the Women's March in Washington, D.C. was approximately three times the size of the crowd that gathered for Trump's inauguration the day before.

Marcel Altenburg and Keith Still, the scientists from Manchester Metropolitan University, estimated that 470,000 people marched on Saturday, compared to 160,000 who attended Friday's swearing-in ceremony. Pressman and Chenoweth's estimate for the Women's March in DC reported similar turnout of about half a million.

Leanna Garfield/Business Insider

Protestors at the Women's March on Washington, January 21, 2017.

Current data from the DC Metro also indicates that the Women's March in DC outpaced the turnout of Trump's inauguration. More than a million trips were taken on Saturday, compared to 570,000 taken on Friday, according to a Metro official. The number of trips on Saturday came second only to the number of trips taken on the day of Barack Obama's 2009 inauguration, the official said, according to the New York Times.

In compiling their data, Pressman and Chenoweth relied on a combination of media and local television reports, police department estimates, and social media.

"In every case we can, we're ideally looking for some kind of print or media source or a police department report that verifies the crowd size," Pressman said, adding that they also relied on photographic evidence for smaller marches.

"Once the ball got rolling, people kept sending photographs in and tweeting at me and [Chenoweth]," he said, while acknowledging that there was room for error in self-reported numbers.

"There have been a lot of people who have reached out to me to contest one of our estimates, and more often than not, it's because they think our numbers are too low," said Pressman. "But I'm more inclined to stay towards the conservative end."

Using Pressman's conservative estimate of 3,220,305 protesters means that almost 1 out of every 100 Americans turned out to march against Trump on Saturday. Using the high estimate of 4,748,992 marchers would place the number at around 1.4% of Americans.

"I was surprised by the totals," Pressman said. "More than that, I was surprised by the geographic scope of the marches. I expected it on the coasts, but it was interesting to see them spread out all over the country."