The Women’s March on Washington has drawn huge crowds of men and women alike to Washington, DC, Saturday.

How big is the crowd? Judging by aerial photos, probably bigger than yesterday’s inauguration of President Donald Trump.

The Associated Press is reporting that at least 500,000 are in attendance — and that estimate may rise as people continue to make their way downtown to the march.

Here’s an image of the National Mall during Trump’s inaugural address, taken from the National Park Service’s EarthCam on the Washington Monument at 12:09 pm ET on Friday, right around when the swearing-in began:

And here’s the Mall today at noon ET, about one hour before the Women’s March was scheduled to start.

Attendance of the Women’s March is in some ways harder to gauge, at least using aerial photos, because many of the marchers are not just on the Mall but also lining Independence Avenue, where they’ll be marching west toward the Washington Monument.

But from what we can tell from the images we have so far, there are many more people on the Mall and the surrounding streets today than yesterday.

The inaugural committee told CNN that about 250,000 tickets to President Trump’s ceremony were distributed, and that additional onlookers were in attendance on the Mall yesterday. But turnout appears to be much lower than the estimated 1.8 million people who came out for President Obama’s inauguration in 2009.

How did attendance at the Women’s March surpass attendance at Trump’s inauguration?

The Women’s March began with a post on Facebook that went viral, drawing together a broad array of women and men of all backgrounds around the idea of demonstrating the day after Trump’s inauguration.

The road to the march hasn’t been without setbacks, however — ranging from chaotic communication to criticism that it wasn’t taking diversity and inclusiveness seriously, or was taking it too seriously.

Organizers received a permit from DC police for at least 200,000 people, but many, including Vox’s Emily Crockett, had predicted that attendance would be higher. Earlier this week, the Washington Post reported that at least 1,200 buses sought parking permits for January 21, compared with only 200 parking permits for January 20.

We may not know for a while which event officially drew larger crowds. Since the National Park Service no longer provides estimates (more on that later), official numbers must be sourced ad hoc from a variety of local DC agencies and federal agencies.

In one attempt to rate the size of yesterday’s inauguration crowd, WMATA, the transit system for the DC area, tweeted ridership stats on Obama’s inaugurations in 2013 and 2009, and President George W. Bush’s inauguration in 2005, as well as Trump’s inauguration. Yesterday’s was the lowest, at 193,000 trips.

Metro Ridership: As of 11am, 193k trips taken so far today. (11am 1/20/13 = 317k, 11am 1/20/09 = 513k, 11am 1/20/05 = 197k) #wmata — Metro (@wmata) January 20, 2017

This morning, WMATA reported nearly 100,000 more rides taken today than yesterday:

Metro Ridership as of 11am: 275k. For comparison, that's more than 8x a normal Sat & even busier than most weekdays. #wmata #womensmarch — Metro (@wmata) January 21, 2017

There is a long history of mass civic protest in Washington, DC. But determining which protests drew the biggest crowds is contentious.

As the nation’s capital, DC has hosted a number of huge protests over the years. And the task of estimating their size often gets contentious.

As Steve Doig, a professor of journalism at Arizona State University who has years of experience estimating crowd sizes, cautioned me, “There is a huge incentive to use the size of the crowd as a token for how important and wonderful [political events are].”

One of the earliest — and at the time, largest — demonstrations was held in 1913 the day before President Woodrow Wilson’s inauguration. Some 5,000 suffragettes and supporters of women’s rights marched on Washington to advocate for equal voting rights for women.

Fifty years later, in the summer of 1963, a crowd of at least 250,000 people gathered on the Mall in front of the Lincoln Memorial to hear Martin Luther King Jr.'s famous “I Have a Dream” speech.

Throughout the 1960s there were many anti-Vietnam marches on Washington calling for the end of the Vietnam War, but the largest of these protests was held in November 1969, drawing at least 600,000 people.

Was this 1969 anti-Vietnam protest the largest in our nation’s capital? Some say it was.

But the Million Man March held in 1995 is also thought to have attracted anywhere from at least 600,000 people to just over 1 million. Organized by Louis Farrakhan and the Nation of Islam, the Million Man March sought to challenge America’s misconceptions of black male identity in the US.

But it also showed just how different crowd estimates can be. The Nation of Islam sharply disputed the National Park Service’s crowd estimate of only 400,000, prompting the agency to stop officially counting crowds at large events on the Mall.

In recent years, estimates have grown even murkier — with more at stake, few want to be responsible for calculating them.

For instance, in 1997 hundreds of thousands of men who were members of the Promise Keepers, a Christian men’s organization that champions conservative values, marched on Washington. But neither the National Park Service nor the event’s organizers were willing to estimate how many men were in attendance.

Counting crowds has never been an exact science, but as Charles Seife, a professor of journalism at New York University and mathematician who has written about the perils of counting crowds, told me, inflated projections can run amok when it comes to calculating attendance.

“The best way is by aerial photography ... take pictures and count heads, but even that is fraught,” said Seife. “When there is a large crowd gathering, someone is going to have a vested interest in having them look larger than they actually are.”

This is good advice to keep in mind as we move into what may be a period of greater political activism, discontent, and public protest.

Watch: Millions join pro-women, anti-Trump rallies