The Blues supporter can be seen goading a group of West Ham fans before shouting and squaring up to an older Hammers supporter.
But seconds later another man steps in to brutally floor him with a single punch.
The incident sparked a wild reaction from a number of bystanders, who shouted and whooped before the man was attended to by two others as he lay motionless on the floor.
He was quickly placed into the recovery position while chants of ‘Irons, Irons’ were heard from home fans in the background.
The Chelsea fan looked almost lifeless after the knockout blow but was seen in a wheelchair later on, despite still looking dazed.
Cops confirmed that a 22-year-old man was arrested after another video on Twitter appeared to show a man being led away by officers.
A Metropolitan Police spokesman said: “A 22-year-old man was arrested before kick-off at the London Stadium today after an alleged assault on another man. The man has been taken into custody.”
Chelsea star Reece James mentioned the attack after the game on Instagram.
Posting on his story he said: “The news has crossed my attention that a fan was punched and knocked out before today’s game.
“I totally understand rivalry and the tension between teams in big games but violence is no answer.
“I hope he’s okay. Appreciate the travelling fans as always. Stick with us and stay safe. See you in Dortmund.”