Detectives are hunting a suspected thug who appears to knock out a passenger during a heated row on the London Overground.
A video circulating online shows the two men having an argument on the service between Clapham Junction and Dalston Junction.
Horrified onlookers attempt to calm the situation as the younger man, wearing a checked shirt over a yellow T-shirt, initially tells the middle-aged male to sit down.
When the victim - who says he lives in east London - asks, “You know where I’m from?”, the alleged attacker replies: “Do I look like I give a f***? You’re in South [London], remember that!”
Violence flares when the older man, in a white T-shirt and dark shorts, calls the aggressor “Afroman” near Denmark Hill station in south London.
A woman is heard pleading: “It’s not worth it, babe.”
The pair square up to each other before a flurry of punches are traded.
The victim falls to the train floor and appears to be unconscious.
Others passengers repeatedly scream “Relax!” as the confrontation stops.
The minute-long video posted on X, formerly Twitter, ends before it is known what happened to either man.
On Monday, British Transport Police said: “Officers are appealing for the victims or any witnesses following an altercation on board a London Overground train to contact them.
“A video has circulated online showing two people in an altercation.
“It is believed this has taken place on the London Overground line between Clapham Junction and Dalston Junction and was believed to be near Denmark Hill Station.
“If you witnessed the incident or if it happened to you, please contact us and our officers will be able to help.”
Anyone with information can contact BTP by texting 61016, or by calling 0800 40 50 40, quoting reference 237 of 21 July.