Looks like https://ash.harvard.edu/programs/crowd-counting-consortium/ would be a good resolution source. They update monthly.

Crowd Counting Consortium – Ash Center

Publicly available data on political crowds in the United States, including marches, protests, demonstrations, riots, and other actions.

https://www.democracynow.org/2025/4/7/hands_off_protest says "over one million" and estimates 100,000 in NYC

“Hands Off!”: 1 Million Protest Trump’s Cuts, Attacks on Education, Immigration, War on Gaza & More

An estimated 1 million protested across the United States and around the world Saturday to tell President Donald Trump and his billionaire ally Elon Musk “Hands Off!” They rallied in opposition to the Trump administration’s dismantling of federal agencies and programs, the war in Gaza and attacks on LGBTQ+ people, immigrants, education, healthcare and reproductive rights. We hear voices from the coordinated “Hands Off!” nationwide protests, described as the largest demonstrations to date since Trump returned to office.

Perfect seismograph market.

It's unclear to me whether the turnout has to be in a single location.

@Kynakwado Not if the Women's march qualifies, which it unequivocally does.

@JamesBaker3 Yeah the Women's March was neither all in one spot, nor 100% marching, so it's pretty clear that those concerns from the comments won't be an issue.

@DavidFWatson

I think this is already yes: https://chatgpt.com/share/67f486d0-1438-800c-85bc-c8b55ea6993a

@DavidFWatson ChatGPT is not a reliable source. Did you read any of the articles it linked?

@jumpman_folder I looked at the cited Wikipedia article, The number of cited cities, and looked at crowd size pictures for a few. Shall we try to add them all up, I agree that’s a reasonable follow up.

@DavidFWatson I made 2 estimates where there were relatively clear aerial pictures

https://bsky.app/profile/siebepersists.bsky.social/post/3lm66tnvesc25

Siebe (@siebepersists.bsky.social)

A couple of crowd size amateur estimates for the #handsoff protests 🧵 Boston, Massachusetts 50K - 63K

@Siebe those are about 1% of their metro populations. I see 144 cities on the Wikipedia list. That might cover 250 million people, leading us to 2.5 million protesters. This doesn’t account for variations in partisanship though. (I haven’t had time to further check individual protests obviously)

@DavidFWatson I appreciate the attempt at an estimate, but this doesn't take into account selection bias - these are probably the biggest protests proportional their population. For example, this implies a 200K protest in New York, which definitely didn't happen

@Siebe fair enough, the nyc one looks a fair bit smaller

@Siebe FWIW this article is estimating 25k for St. Paul: https://kstp.com/kstp-news/top-news/thousands-of-people-gather-at-state-capitol-for-hands-off-rally/

@jumpman_folder thanks, that would imply a much lower density, or the map tool being wrong. I did mark the area right..

Are people betting this down because there are going to be at least two of these protests, and they want to argue that that isn't "an" anti-Trump protest?

@Panfilo the fact that this hasn’t resolved “yes” yet might indicate that something screwy is going on with the author’s beliefs about what qualifies

@DavidFWatson I'd guess an official reliable number still seems needed. When is the second protest?

@AndrewMcKnight that’s not what the market description requires. consensus/Wikipedia is sufficient. Why does there need to be a second one?

@Panfilo I dumped my YES shares because CNN and other mainstream publications switched from reporting “millions of people” to “scores of people”

@jumpman_folder Damn guess it was only 60 or 80 or so folks

"Hands off" protests reporting should be checked, I think it might resolve this YES?

@JoshuaPhillipsLivingReaso https://www.cnn.com/2025/04/05/us/hands-off-protests-trump-musk/index.html

CNN is reporting "millions of people".

@Twig Oh - but the Hands Off protests are not a march. Subtle!

@dlin007 What do you mean by "March" in this question? Or is any protest sufficient?

@Twig any protest is sufficient, yes. the women's march was just an example of what would qualify

@dlin007 Thank you!

Wikipedia now cites claims of 3 million for Hands Off, from both organizers directly and organizers via CNN, though it does not yet make the attendance claim itself.

@Twig The CNN article is simply quoting the organizers. They didn't verify the attendance.

@TimothyJohnson5c16 That is correct. I regret not making this more clear, and have edited my comment now. I appreciate the correction!

@JoshuaPhillipsLivingReaso this is going to be tricky to get a reliable estimate on!