DOWNTOWN — A video going viral Friday shows a man being punched and falling onto the Blue Line tracks, allegedly for using a racial slur during an “L” platform altercation.

The incident happened about 2 p.m. Thursday at the Clark/Lake Blue Line station, according to a witness. The video gained attention on Thursday night, when it was widely shared on Facebook and Twitter, racking up thousands of views.

The video shows two men who appear to be arguing, and someone can be heard saying the n-word. A man then slowly unbuttons and removes his dress shirt, puts his glasses in his pocket and punches the other man, knocking him down.

The man who was hit falls down as people shout.

“Call another motherf—– a n—–, b—-,” someone can be heard saying. “F— wrong with you?”

The punched man appears to try to stand but falls onto the tracks. Several people help the man off the tracks and the video ends.

Mike Kemper, a Ukrainian Village resident, said he witnessed the incident and heard the punched man use several racial slurs before he was hit. When the man was knocked onto the tracks, a black man came over to help him up — but the punched man then yelled racial slurs at the helper and “pretty much anybody he could,” Kemper said.

Someone who appeared to be the man’s friend “dragged” him away from the situation, Kemper said. The man appeared intoxicated, Kemper said.

Bystanders spoke to the two black men — including the one who punched the man — who had been the target of the man’s slurs, asking if they were OK and trying to calm them down.

“Everybody was pretty much on their side,” Kemper said. “Everybody pretty much just made they were all right [after] getting verbally abused and whatnot.”

People told the men, “‘That was kind of crazy. What you did was justified.’ He was being insanely racist to everybody,” Kemper said.

Kemper saw a police officer speaking to a CTA employee, but he got onto a train and left shortly after the incident.

The video has incited controversy online, with viewers debating if the man used the n-word and, if he did, if the punch was a justifiable response.

A person who posted the video said on Twitter that the man who was punched had called a black man the n-word and “charged” at a black woman before being hit.

“Racist in #Chicago drops the n-bomb on brotha minding his business and catches the Blue Line to FadeTown,” the poster wrote, adding the hashtags, #TrumpsAmerica, #PunchANazi2018 and #Resist, among others. His post of the video has been watched more than 625,000 times.

But that poster said he hadn’t shot the video. Block Club Chicago contacted him and others who shared the video but was unable to immediately find the original source.

The CTA referred inquires about the fight to the Chicago Police Department, but a police spokesman said he could not find record of the incident.