A man was in critical condition Thursday night after being shot in the head on a subway train as it arrived at the Hoyt-Schermerhorn station in Downtown Brooklyn during the Thursday evening rush hour, the authorities said.

The shooting occurred after a 32-year-old man boarded a northbound A train at the Nostrand Avenue stop at about 4:45 p.m., Michael Kemper, the Police Department’s Chief of Transit, said at a news conference.

As the train left the station, the man was confronted by a 36-year-old man who witnesses described as acting in an “aggressive and provocative” manner, Chief Kemper said.

What started as a verbal confrontation quickly become a physical fight, the chief said, with the 36-year-old man displaying either a knife or razor blade at one point. Eventually, he pulled out a gun, Chief Kemper said.

The men began grappling, and the 32-year-old man grabbed the gun and shot the other man several times, Chief Kemper said. He said he did not believe the two men knew each other.

Footage of the fight posted to social media shows that the 36-year-old man repeatedly threatens the 32-year-old man, standing over him and saying “I will beat you up” before the younger man stands up and the two circle around each other. As the fight escalates, other passengers move to the opposite end of the train, but one woman stays back.

The video shows that as the two men tussle on a pair of subway seats, the woman also becomes involved, appearing to fumble in her purse for something and then stab the 36-year-old in the lower back as he stands over the younger man, pummeling him. Another rider, wearing a blue sweatshirt and a yellow neon vest, attempts to intervene and briefly separates the men before the fight resumes.

As a bloodstain grows on the 36-year-old man’s white T-shirt from the apparent stab wound, he asks, “Did you stab me?” The 32-year-old man then takes a protective stance in front of the woman, and the two of them are backed against the end of the subway car. The 36-year-old man fishes around in his jacket — which he had thrown onto a seat — and produces a gun as he repeatedly yells, “You stabbed me, right?”

The other riders, including the person recording the video, run farther back into the car and hit the ground. As the riders panic, the last glimpse of the 32-year-old man shows him trying to cover the woman with his body. The video does not capture either of the men as the train pulls into the station and the doors open, but shots ring out. Riders run in all directions looking for cover. Four shots can be heard in the video.

Police officers at the Hoyt-Schermerhorn station heard gunshots as the train arrived there and responded immediately, Chief Kemper said. The man who was shot was taken to a hospital, where he was undergoing surgery and was in critical condition, the chief said.

The authorities had not determined by Thursday evening whether to charge the 32-year-old or whether his actions would be considered self-defense, Chief Kemper said.

Lincoln Restler, the City Council member who represents the area, called the shooting “a horrible tragedy.”

“I know it’s unnerving for the four million people who ride the subway every day when a terrible, violent incident like this occurs,” Mr. Restler said.

The shooting occurred amid new scrutiny of the subway’s safety after a recent a recent spate of violent incidents. In January, a 45-year-old crossing guard was shot dead on a train in Brooklyn after intervening in a fight and in February, a 35-year-old man was killed and five other people were wounded after an argument broke out among teenagers on a train in the Bronx.

Last week, Gov. Kathy Hochul deployed hundreds of National Guard members and state troopers to check riders’ bags in an effort to deter crime and, she said, to ease New Yorkers’ fears about the system.

Surveys by the Metropolitan Transportation Authority, which operates the subway, consistently show that some riders perceive the subway as dangerous, but data does not necessarily confirm that view. There has not been a significant increase in crime in the system recently, and the likelihood of a person being a victim of a violent crime is remote.

In mid-2022, there was about one violent crime per million rides, according to a New York Times analysis. Since then, the overall crime rate has fallen and ridership has increased.

The Hoyt-Schermerhorn station, which is served by the A, C and G trains, contains a police outpost, Transit District 30. For about two hours, all trains bypassed the station as the investigation proceeded and the police advised the public to avoid the stop. But by 7 p.m., the station had reopened, with trains running in both directions.

Several videos posted on the social media site X by a journalist, Joyce Philippe, showed the shooting’s aftermath: a crowded A train stopped in the station as shouting is heard from the platform, and people ducking for cover at the center of a subway car while others cry in the background. At one point, some riders shouted for the conductor to close the train doors.

“Where’s the N.Y.P.D.? Oh, my God,” one person can be heard saying in one of the videos.

Another video showed police officers on the platform, at least one of them with his gun drawn.

“The victim here, as the chief said, appears to be the aggressor,” Janno Lieber, the transit authority’s chief executive, said at the news conference. “But the real victims are the people I saw in those videos, who are having a harrowing time because they’re on a train with somebody with a gun.”

Natgel Maldonado, a 16-year-old student, said he had been waiting for a southbound train at the Hoyt-Schermerhorn station when a northbound A train pulled in and a chaotic scene unfolded. He described people fleeing from the train and toward the station exit. It was unnerving, he said.

“I’m 16 years old and I’ve lived in New York all my life,” he said. “This is the first time something like this has happened.”

Ana Ley, Eliza Fawcett, Sean Piccoli and Chelsia Rose Marcius contributed reporting.