A video uploaded to social media appears to show hooligans squaring up for an organised brawl.
It is claimed the fight is between youth firms connected to the two London-based Championship rivals from Millwall and Brentford far from any stadium.
The three-minute video shows the men - half of them topless - meeting on a muddy field surrounded by graffiti-strewn walls.
The event is said to have occurred on August 25th, however it was only uploaded to Facebook and YouTube on Monday.
It's unclear where the fight took place, but a spokesperson for the Metropolitan Police told Mirror Online they are looking into the footage.