When the conspirators against the king met for a strategy session, the blowhard sorcerer boasted that he could call spirits from the vasty deep. “Why, so can I,” said his testy partner, “or so can any man. But will they come when you do call for them?”

When an influencer calls his followers, they come; that is why he is called an influencer. Kai Cenat is a 21-year-old whose online ventures earn him over a quarter million dollars a year. He makes that from the social-media platforms to which he brings fans via raps, comic stunts and sketches, interviews with celebrities (Lil Baby, 21 Savage), and videos of him falling asleep (this genre is known as sleep-streaming). One day this month he announced he would give away video-game consoles in Union Square the following afternoon.

Union Square takes its name from the intersection of Broadway, originally a colonial and Indian path, which drifts across the island we inhabit from lower right to upper left, and the city’s 19th-century street grid. Now Union Square is also the intersection of subway lines: the 4, 5, 6, L, N, Q, R, and W. Anyone who can hop a train can get there. By 4 p.m. on the day of Cenat’s giveaway, 6,000 of his fans had gotten there.

We first noticed something afoot by the number of flashing lights on the avenue 13 floors below us. There is always a police car or an ambulance about, hurrying or parked, but now there were six stationed in one block, three on each side of the street. At the same time, we became aware of the steady din of helicopters — not one lazily noting traffic patterns, but a cluster hovering in place. It was time for our afternoon constitutional, to which was now added the incentive of inquiry. A neighbor in the lobby gave a warning in the form of a bulletin: crowd — giveaway — riot — don’t go. Word made us simultaneously more cautious and more curious. Outside, there was an unusual number of teenagers about, all black, mostly looking puzzled. Across the way, a platoon of police walked square-ward, unarmored, but each carrying a plastic shield. We adjourned the constitutional sine die.

Back upstairs and online — for the modern experience of seeing what you have experienced explained on a small screen — tweeted clips unspooled. Union Square is a rectangular park, three blocks long, one wide. When I moved to the city its grass had been pounded to dirt and its bushes harbored druggies offering smokes. The trees above shook their heads at the waste. Now they shade grass, plantings, and benches, with a dog run in one corner. The north end of the park has a playground and a restaurant in an old band shell. The south end is an open semicircular lip, used by performers, skateboarders, people sitting, and chess players. Young men in uniform, African immigrants by the look of them, keep it swept. Washington, Lafayette, and Lincoln give bronze civic lessons. Several days a week a farmers’ market doglegs around two sides. Now it was pandemonium. Someone offered free stuff. People came. But neither he nor it was there. (Cenat did make a brief appearance but was whisked away in an SUV with the besotted clinging to the roof as to a train in Pakistan.) What to do? Many in the crowd, perhaps most, had no idea, and stood or wandered around. Such were the scattered kids outside our door. But troublemakers made trouble, and spectators expecting a show decided to join in and make their own. Kids fought, threw stuff at each other and at cops. They bounced up and down on the roofs of cars unfortunate enough to be caught in the press. They clambered up lamp poles and atop the nouveaux beaux arts roofs of subway entrances. They trashed the carts of food vendors, all Muslim men, emptying the tills and stomping on the roofs. Many filmed themselves, or were filmed by others. They might be the next influencers! Time, and a steady supply of police reinforcements, eventually restored order. Seven injuries were reported; a report of a gunshot wound turned out to be false. Sixty-six people were arrested.

One of those was Mr. Cenat, who was charged with inciting a riot. (NB: If he was, why not the 45th president?) I hope some ambitious lawyers will represent the robbed and pummeled food vendors in civil suits. There are pockets to be lightened.

Haters of the city took to the ether, framing the event with scare verbs: The park was “trashed,” etc., as if it had suffered a Hawaiian wildfire. Thank God, the trees are old and the statues hard. No life was lost. You would not know to see it now that anything amiss had happened. But the prospects for a next thing are disturbing. The great weapon of the city’s police when crime was rolled back during the Nineties was COMPSTAT, the brainchild of Jack Maple, originally a lieutenant in the transit police. Maple (d. 2001) was an eccentric who favored homburgs and spats. Drawing perhaps on his visual sense, he found that when crimes were mapped on a daily basis, patterns emerged that allowed crime fighters to respond and, better, to anticipate. COMPSTAT = computer statistics, the means of his method. That was tech eons ago (Maple actually started by sticking pins in wall maps). Now everyone carries a computer in his hand. When Mr. Hustler says ready, hordes get set and go. As in warfare, advances in weaponry are surpassed by countermeasures. Those who guard us while we sleep will have to pay more heed to sleep-streamers.