A 19-year-old in a green jacket and ripped jeans held his hands above his head and sat alone in the middle of a New York subway car on Friday.

As the other passengers watched, nearly a dozen police officers peered in from the Franklin Avenue station platform in Brooklyn.

Then, one by one, they noticed that several officers were pointing guns into the crowded afternoon car at the unarmed black teen. Some yelled in horror as they squeezed together on either end of the car.

Adrian Napier kept his hands in the air and asked the officers whether he should stay in the seat or lie on the floor.

“Call my mom,” he told a someone on the train.

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Seconds later, a group of New York police officers flooded the train and tackled the young man to the ground, cuffing and frisking him.

The officers did not find the gun they were looking for, but they arrested Mr Napier for fare evasion, charging him with theft of services for jumping over a turnstile.

The tense encounter was caught on film by another passenger on the train, who posted the video on Twitter where it has been viewed more than 3 million times since Friday evening.

“After that one policeman took his gun out, two or three more took them out,” Elad Nehorai, who shared the video, told The Washington Post on Monday.

“For a moment, they were kind of pointing guns at everyone who was in that vicinity.”

The former housing secretary and Democratic presidential hopeful Julián Castro criticised the officers for responding with guns to a man who was ultimately charged with stealing a one-stop ride on public transportation.

“Officers should be working to deescalate – not putting dozens of lives at risk over $2.75,” Mr Castro said on Twitter.

The New York Police Department said in a statement shared with The Washington Post on Monday that a witness near Atlantic and Flatbush avenues in Brooklyn told officers that Mr Napier had a gun around 4.40pm on Friday.

NYPD said officers first tried to approach the 19-year-old a short time later but he fled into the Pacific Street subway station.

Mr Napier allegedly slipped past the turnstile and boarded a southbound train. At least 10 officers were waiting at the Franklin Avenue station when the train pulled in.

“Officers at the Franklin Avenue station located and identified the individual inside of the train and took him into custody without incident,” NYPD said. “The individual was arrested for not paying the fare and was charged with Theft of Service.”

NYPD did not immediately return a request for information about its training or protocols for making arrests inside subway trains.

The NYC Civilian Complaint Review Board, which investigates allegations of police misconduct, contacted Mr Nehorai on Sunday and told him how to file a complaint against the NYPD officers.

“The CCRB takes allegations of police misconduct seriously,” the independent oversight agency said in a tweet.

Other passengers on the train were visibly shaken by the arrest. Several people shouted warnings about the officers’ guns. Others flinched at the sight of the barrels.

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Within a minute of stopping in the station, everyone else had moved from the centre of the car to the edges, leaving the 19-year-old alone in the line of fire.

“It was a combination of feeling incredibly concerned for [Mr Napier] and also being aware that we were in danger,” Mr Nehorai told The Post. “If anything small went wrong, someone could have been hurt.”

Mr Nehorai, a marketer who organises a liberal activist group for Orthodox Jews, said he was more afraid of the police than the suspect, even though he initially thought the young man was being arrested for a serious crime.

“Someone mentioned they thought [Mr Napier] was the one who kept us the safest because he didn’t do anything and he was very careful,” he said. “He didn’t try to hide, he didn’t try to run.”

One woman said on Twitter her young sons were riding the train when police pointed their service weapons into the subway car. Yolanda Sangweni said her 10-year-old came home “inconsolable” after seeing the arrest.

“How do we, as parents, even begin to speak to them about this kind of violence?” she said. “You see the trauma we’re exposed to at a young age.”

Police critics latched onto the video to slam the plan to hire more fare enforcement-focused police officers and condemned police for escalating the situation.

“2 minutes 20 seconds. Of terror,” Scott Hechinger, director of policy at Brooklyn Defender Services, said in a tweet. “All to arrest a young man sitting down, his hands in the air, doing nothing. This is a militarised police out of control. And they hired 500 more for the subway.”

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New York police were rebuked again by some observers for a violent incident this weekend that was also filmed and posted on social media.

NYPD officers appear to brawl with several people after responding to a call about two groups of people fighting on a subway platform. Officers exchanged blows, striking people in the face and tackling them to the ground.

“Officers responded to a fight between two large groups, during which individuals began to interfere with police action and the situation escalated,” NYPD said in a statement shared with The Post. “Publicly available video does not show the entirety of the incident, and the department is conducting a review.”

Critics argued that the aggressive police actions could have escalated both incidents. There have been 11 fatal shootings by NYPD officers so far this year, including six in the last month.

“If this is how things go ‘well’ – when someone is not killed or hurt – it’s not a far leap to understand why so many people do get killed, who are innocent or at least not doing anything they should be killed for,” Mr Nehorai said.