Former President Donald J. Trump claimed without evidence on Sunday that his rival in the 2024 presidential election, Vice President Kamala Harris, had used artificial intelligence to doctor or create an image of a rally that showed a large crowd in Detroit last week.

“She ‘A.I.’d’ it, and showed a massive ‘crowd’ of so-called followers, BUT THEY DIDN’T EXIST!” Mr. Trump wrote on his social networking site, Truth Social.

Three reporters from The New York Times who attended the Michigan rally confirmed that the crowd numbered in the thousands, contrary to Mr. Trump’s assertion that “there was nobody there.” A Times analysis of photos and videos of the event also showed that Mr. Trump’s claims about the size of the crowd were unfounded. Other images and videos from multiple vantage points showed a large audience.

Crowd size is often a point of comparison between candidates, but Mr. Trump has now given it more intense focus.

A Harris campaign official told The Times by email that the original photo in question was taken by a campaign staff member and was not modified by artificial intelligence.

The crowd was packed tightly inside the airplane hangar. Some attendees waited for Ms. Harris’s arrival on elevated platforms that gave them a higher vantage point, but a majority stood on the floor. From some angles at the back of the hangar where reporters were positioned, it was difficult to gauge how far the crowd extended.

But by moving over to a nearby riser that faced the open door of the hangar, Times reporters could see the size of the crowd more clearly. It stretched beyond the hangar, spilling out onto the tarmac beyond — not far from where Air Force Two came to a stop.

Ms. Harris’s X account streamed the rally, with the first few minutes panning over the crowd as Air Force Two arrived. The Harris campaign later posted on X that the image reflected what the campaign said was 15,000 attendees, and it also responded to Trump’s post with a video of the crowds as Air Force Two arrived at the Detroit Metro Airport.

Posts calling into question the authenticity of the crowd photo, and of the size of Ms. Harris’s audience, had already begun to bubble up among far-right Trump supporters before Mr. Trump seized on it. Mr. Trump’s Truth Social post included a screenshot of an X post by Chuck Callesto, calling the crowd image “FAKE.”

Mr. Callesto is a conservative social media strategist who has frequently posted falsehoods and “Stop the Steal” content about the 2020 election. Laura Loomer, a right-wing activist whom Mr. Trump wanted to hire for a role on his campaign, posted multiple times about the photo, and Joe Hoft, a far-right blogger, wrote a post about it.

Before Ms. Harris’s rally, Mr. Trump had already been focused on comparing crowd sizes at her rallies with those at his own. He said during one of his campaign speeches that Ms. Harris had drawn crowds because she had entertainment and that he did not need to do the same to attract attendees.