Kai Cenat was live online last week when he began planning himself a homecoming.

“We’re going to be going crazy,” Mr. Cenat, a social media superstar who has become known for his marathon streaming sessions, said during a Wednesday broadcast on the social media platform Twitch. He announced a gathering in Union Square to tens of thousands of viewers. All his New York people needed to be there, he said.

He encouraged the audience to arrive on time, adding that the event “might end really quick, depending on how rowdy” it got.

It got rowdy. Two days later, thousands of young people showed up in Union Square, some of them jumping on cars and vans, many with their phones out and filming. The crowd chanted obscenities, threw soda cans and trash at the police, and set off fireworks and car alarms.

Mr. Cenat was arrested — along with at least 65 other people — and charged by the police with inciting a riot. Nearly half of those arrested were under 18.

New York is a city of crowds, of spectacle. Few who live in the city think twice when a horde of people suddenly appears out of thin air; many just walk quickly in the other direction.

But Mr. Cenat’s ability to summon a legion of eager teenagers startled even the most jaded residents. For those unfamiliar with Mr. Cenat — which it’s safe to say includes most over-21 New Yorkers — it was as though full-blown Beatlemania had suddenly materialized for some guy on the Q train.

The Union Square takeover exemplified the power of niche internet celebrity and again demonstrated what can take place when the energy of an online fan base bursts into the non-virtual world, whether on a Canadian sunflower farm, at a 17-year-old’s birthday party, in the U.S. Capitol or the heart of New York City.

“This young man, this influencer, I believe he saw that day how much influence he really has,” said Jeffrey Maddrey, the police’s chief of department, in an interview with Fox 5 New York.

Mr. Cenat, 21, grew up in the Bronx but currently lives in Atlanta, a hub for social media creators with followings on TikTok, YouTube and Twitch, a livestreaming platform. Over the past year, Mr. Cenat has become Twitch royalty: He has 6.5 million followers and in February, broke the platform’s record for the most paying subscribers.

His fans tune in to watch him crack jokes, host celebrities, play games and pranks, invent new slang like “rizz” (that was him) and just hang out. Estimates of his annual income range from the hundreds of thousands of dollars into the millions.

In his Wednesday livestream, he announced that he would give away computers, gaming consoles, microphones and video monitors at Union Square. In short, he would give away the equipment that would allow others to do what he does.

The youth who flocked there on Friday were not necessarily there for free merchandise, the chance to see Mr. Cenat or even to be featured in his stream. At least some were there for the chance to become him. Mr. Cenat himself referred to the event — with some irony — as the “‘stay off the streets and go stream’ project.’”

Keith Dorsey, 36, the founder and chief executive of Young Guns Entertainment, an Atlanta-based media company that manages young people who make content for social media, said in an interview that a generation of Black and brown children saw themselves in Mr. Cenat — and that he had a special attraction for young people in his hometown.

“When you have a regular individual, a regular person, a young person who just goes online and blows up, it reaches them a little differently,” he said.

For years, social platforms have created a crop of celebrities who cultivate followings through incessantly posting videos of themselves: talking, singing, joking, playing games, traveling and even sleeping. (Mr. Cenat, in particular, has been known to zonk out on his livestreams.)

All over the United States, young people — some living in group houses, others solo with just a ring light and a camera — have built followings unlimited by physical proximity. Occasionally, they walk among us, looking like normal teenagers, except they’re often trailed by a camera and in the case of a lucky few, a string of fans.

Social media stars emerge rapidly. Before Mr. Cenat, who was born in December 2001, became a teenager, the social internet was dominated by fleeting video clips on the platform Vine, which sprouted a generation of mini-celebrities who could express themselves in the six seconds available.

When Mr. Cenat was attending high school at Frederick Douglass Academy in Harlem, a YouTuber named Jake Paul began to attract attention, staging media-baiting pranks that made him the bane of his West Hollywood neighborhood.

And by the time Mr. Cenat was in college at the State University of New York at Morrisville, a charismatic gamer who went by the name Ninja had become a superstar on Twitch, with tens of thousands tuning in to watch him play the video game Fortnite.

By then, Mr. Cenat had begun to travel to Atlanta to work with a collective of fellow entertainers that called itself A.M.P., for Any Means Possible.

Mr. Cenat, a good-natured, high-energy performer known for a relentless work ethic, has found success with a set of familiar audience-drawing techniques: He makes prank calls, hosts impromptu trips abroad and sets off fireworks indoors. He has increased his viewership by hosting young rappers including Lil Uzi Vert, Ice Spice and 21 Savage.

And he has begun to indulge in the occasional giveaway — another popular genre of online video. In a July clip, he surprised a Massachusetts woman he used to visit in the summers with $20,000, saying she had been a second mother to him, and she deserved it. (He has iffy luck with giveaways; the woman, Cathy Parker, was reluctant to accept the money.)

Last month, an interviewer for Complex magazine, Speedy Morman, asked him why he had begun livestreaming, saying, “I think I speak for a lot of hip-hop culture in that I really wasn’t familiar with the streaming world. I hadn’t really heard of it until you.”

Mr. Cenat acknowledged his role as a trailblazer. “Our community with Black streamers just isn’t as big as the white streamers,” Mr. Cenat said.

All livestreaming thrives on spontaneity — or at least its appearance — and Mr. Cenat seemed last Wednesday to have done very little planning for the New York giveaway.

He explained the giveaway itself in vague terms, saying fans would win the free gear by correctly answering trivia questions. He said he had chosen to hold the event at 4 p.m. because he felt it would be “calm,” and in Union Square because “all trains go there.”

Then, Mr. Cenat asked his fans to say “me” if they were from New York. As the chat that is visible during his livestreams filled with “me” he began to clap, and then to laugh, seemingly at the sheer number of responses.

Two days later, New Yorkers encountered the me army in person.

Lisa Swan, 56, who works in digital marketing, had a late lunch near Union Square and decided to buy some food at the farmer’s market.

“As I’m walking out of the subway, this was probably around 2:30 in the afternoon, I see these massive crowds,” she said. “And there’s a kid climbing on one of the streetlights. He looked like Spider-Man.”

Ms. Swan stuck around until she saw several people leap onto the roof of the gazebo that marks a subway entrance.

“I said to myself, this is not going to end well,” Ms. Swan said.

The event descended into chaos. The police estimated as many as six thousand people, most of them young, showed up. While most were well-behaved, some were caught gleefully destroying property, including vehicles.

It seems that Mr. Cenat briefly livestreamed from Union Square. A video on one of his YouTube channels shows him entering the crowded park, before cutting to a local news broadcast that shows an SUV driving away, with a number of young people clinging to the roof.

“It became very dangerous very fast,” Mr. Maddrey said in the Fox interview. He said when he had arrived in Union Square he had called his own house to ensure his daughter was not in attendance. “I had to know where she was at,” he said.

Mr. Cenat’s manager forwarded an interview request to his publicists, who did not respond to questions, including whether Mr. Cenat had purchased the equipment he said he would give away, and what, if anything, had happened to it.

The firm did issue an apology on behalf of A.M.P. saying it was “deeply disappointed by the outbreak of disorderly conduct.”

Mr. Dorsey, the chief executive of Young Guns, said that while Mr. Cenat likely had good intentions, he had erred by moving forward with the event as if he were making one of his usual videos.

“He didn’t know it was going to be that big,” Mr. Dorsey speculated. “Within this business, a lot of the creators, they grow really fast. And they don’t know really know what type of influence they have until something like that happens.”

He said that next time, Mr. Cenat’s management team should be sure to organize with the offline authorities. At the same time, he said, cities should respect the power wielded by influencers.

There is likely to be a next time: In the attention economy, celebrities like Mr. Cenat can capitalize on the buzz created by events like the Union Square takeover. Certainly last week, during his livestream, he was already dreaming up more events for his fan base.

“We have a lot of things to do before school starts,” he said.