Brandon Stanton, the nimble shutterbug behind the immensely popular photo blog Humans of New York, has worked hard to filter politics and moral judgments out of his posts, intent on maintaining objectivity as he captures his subjects in words and on film, letting them speak for themselves.

That changed last week when Mr. Stanton, 32, shed his sedulously cultivated neutrality to take on Donald J. Trump, excoriating the Republican presidential candidate in a 300-word Facebook post presented as an open letter to Mr. Trump.

“I’ve watched you retweet racist images,” the post read in part. “I’ve watched you retweet racist lies. I’ve watched you take 48 hours to disavow white supremacy. I’ve watched you joyfully encourage violence, and promise to ‘pay the legal fees’ of those who commit violence on your behalf.”

The reaction was explosive. Within eight hours the post was shared 712,000 times, eventually garnering more than 2.2 million “likes,” 1,131,389 shares and 69,000 comments, making it among the most-shared posts in the history of Facebook. In the process, it turned Mr. Stanton, already a best-selling author, into a web sensation.

Shortly afterward, he was summoned by Katie Couric, Yahoo’s global anchor, to appear as a guest on her web talk show, an occasion he welcomed, telling his host, “I want people to think of this as a moral question, not a political question.”

The clamor had by no means subsided when a boyish, tidily bearded Mr. Stanton arrived the other day at the Starbucks at Ninth Avenue and West 59th Street. Patrons strained for a closer look; one, who identified himself only as Ben, sprang from his chair to approach Mr. Stanton and invite him for coffee. (Mr. Stanton good-naturedly declined.)

Safe then to say that for this apparently reluctant Internet star, life has taken an unforeseeable, and perhaps irreversible, turn.

Mr. Stanton, who has 17 million followers on Facebook (Mr. Trump has 6.7 million) and two best-selling anthologies of his posts, had, for all that, chosen to largely remain an inconspicuous figure, a camera-toting Everyman, his face all but obscured by a hoodie and FDNY baseball cap.

His decision to go public, embracing the role of moral gadfly and guardian of the public weal, came as something of a shock, even to himself.

“I saw my decision as a trade-off between upsetting people and making a strong moral statement,” he said.

The choice, Mr. Stanton said, did not come naturally. “I was nervous,” he said. “But eventually I closed my eyes and hit ‘post.’”

It was the content, not the volume, of the subsequent responses that took him aback. “I thought a lot of the feedback was going to be ‘Stay out of politics,’” he said, leaning forward intently as the conversation picked up steam. In no way had he anticipated its impact or the avalanche of commentary that followed, much of it echoing or expanding upon his own sentiments. (The Trump campaign did not respond to a request for comment for this article.)

It was the sort of validation Mr. Stanton said he has never chased.

“I’m not an activist,” he said. “But in keeping silent about a moral problem, I began to feel a lot of guilt.”

Indeed, his post was strikingly at odds with his customary posture as all-but-invisible observer.

“On the blog I use very few words and no photos of myself,” said Mr. Stanton, who draws his income from book sales and speaking engagements, and says he has turned down millions of dollars in online advertising. “I don’t want to rent out the influence of ‘Humans of New York’ to any third party.”

Still, he’s not a complete stranger to the camera. Viewers may have caught his appearance on CNN last December as a passionate advocate for the refugees of Syria and Iraq, and one in particular, identified only as Aya. Mr. Stanton wept openly in discussing her plight with the host, Fareed Zakaria.

At Starbucks, he projected a similar intensity, reddening from time to time, describing his work with a missionary zeal. He grasped this reporter’s arm repeatedly to hammer home a point, and from time to time clamped a hand over hers, lest she somehow misinterpret him.

“I think there was a power in speaking directly to Donald Trump, rather than writing a think piece about him, ” Mr. Stanton said. “It also gave people who I think feel very strongly about him a chance to speak directly to him.”

His decision arrived by degrees. In the half-dozen years since he left his job as a financial trader in Chicago (not quite voluntarily, as he has made clear in past interviews) to take up his camera on the streets of Manhattan, Mr. Stanton has snapped and interviewed some 10,000 randomly selected strangers, posting their photos and capsule life narratives on his blog.

He is welcomed at times but just as often rebuffed.

“It’s impossible to do this job without a strong sense of humility,” he said. “That’s what it takes to be able to intensely listen as if each person’s viewpoint was as valid as your own.”

He interjected, with a kind of perverse pride, “I’m the only person with a New York Times best seller that gets treated in the street like a homeless person every single day.”

The transition from faceless chronicler of ordinary lives has been, from his perspective, gradual and all but imperceptible.

His political engagement became apparent to some when Mr. Stanton first visited the Middle East.

“I find it fascinating when people state that they ‘try not to be political,’” wrote Mike Riverso, a Canadian, on Twitter. “I’ve always seen everything that Humans of New York does as a political act. Humanizing Syrian refuges at a time when they’re highly vilified is inherently political.”

Mr. Stanton shrugged in acknowledgment. “When you write about people whose lives are infused by politics,” he said, “politics just naturally enters the work.”

In interviewing inmates of the Federal Correctional Institution in Cumberland, Md., he strove to maintain his detachment. At the time, “I never brought up issues related to the criminal justice system,” Mr. Stanton said. “They came up in conversation just the same.”

He said he hoped eventually to return to his role as a somewhat inconspicuous outsider.

“Politics became my life for two days,” he said. “I’m trying to get down from that as soon as possible, so I can focus on my day-to-day work.”

He may or may not continue to monitor Mr. Trump.

“Let’s see what happens,” Mr. Stanton said.