A group of Philadelphian train passengers held down an alleged gunman after a Thursday shooting incident until police arrived, video shows.

A 19-year-old was shot in his left arm on a Southeastern Pennsylvania Transportation Authority (SEPTA) train just before 8 a.m. The incident spilled onto the 52nd Street Station platform in West Philadelphia, with a few people detaining the suspected shooter before police arrived.

Cellphone footage captured by witness Mark Krull shows Market-Frankford Line passengers holding the suspect down until police arrived on the scene.

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SEPTA passengers holding suspect down

Cellphone footage captured by witness Mark Krull shows SEPTA passengers holding the suspect down before police arrived on the scene. (Mark Krull)

"Keep the kids down, keep the kids down," one man is heard saying in the video.

"It looked like they were beating somebody up. You hear a muffled gun shot and we all ran out of the train and hid behind one of those blue storage things SEPTA has," Krull told FOX 29 Philadelphia. "I was a bit shaken up. I ride the train all the time and it's usually not violent."

The victim is in stable condition. Police said that the shooting started with an argument between two male passengers.

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Police arresting SEPTA shooting suspect

The suspect was arrested by police, who say that the 19-year-old victim is in stable condition. (Mark Krull)

"That altercation was quickly joined by two additional males, erupting into a full-blown three to one assault," Acting Chief Chuck Lawson said.

Authorities said that three men were taken into custody, with a fourth suspect still sought by police.

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Shooting on SEPTA train

The same men who detained the suspect on the train platform also held him down on the SEPTA train. (Mark Krull)

Fox News Digital reached out to the Philadelphia Police Department for a statement, but no new details are available.