The image, snapped by Barbara Kinney, a photographer on the Clinton campaign, was posted on Twitter, where it was shared more than 8,000 times in a matter of hours and drew hundreds of comments.
"Wow millennials really do hate Hillary," one commented, tongue in cheek.
Others suspected Clinton orchestrated the photo-op herself. "I'm assuming she asked people to take selfies? Almost Trump-class on that kind of social media strategy," one tweeted.
Another commented: "This is so strange that I'm wondering if she invited them to all do it at once."
Clinton got in on the act later, taking a selfie herself with some supporters.
The selfie phenomenon has created some memorable snaps. There was the one when the Pope snapped himself and some visiting schoolchildren at the Vatican.