The two teams from the capital were held to a stalemate in a 1-1 draw in the Premier League clash.
But there is no love lost between the two rivals and trouble had flared up even before the game began outside the London stadium.
Footage shared on Twitter appeared to show a Chelsea fan engaging in a verbal confrontation with a group of people.
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.”
Joao Felix, sent off on his debut, returned from suspension to give the visitors an early lead from a cross by £107m man Enzo Fernandez.
But ex-Stamford Bridge left back Emerson equalised for the Hammers on another frustrating day for Blues boss Graham Potter.
Soucek thought he had stolen all three points for the Hammers when he forced the ball home after Kepa Arrizabalaga had saved Rice’s header.
But the Czech was ruled offside after a VAR review which saw the game end honours even.
It was another disappointing day for big-spending Chelsea as they failed to take a grip of a humdrum game.
Potter has now overseen a run of just two wins in 13 Premier league games
Marc Cucurella was booed off by Chelsea fans against West Ham.
The left-back was taken off in the second half for Ben Chilwell as the Blues went in search for a winner.