West Ham captain Mark Noble shoved a pitch invader to the ground as frustrations boiled over in their match against Burnley at the London Stadium.
Moments after Ashley Barnes gave the visitors the lead, a man ran onto the pitch from the home end.
With no stewards on hand to remove the fan, an enraged Noble grabbed him and threw him to the floor.
Two more fans then encroached the pitch and were led away by Hammers defender James Collins.
And, after Chris Wood put Burnley two up, another fan picked up the corner flag, marched to the centre circle and planted it in the ground.
Meanwhile, fans gathered below the directors' box chanting 'sack the board' as the atmosphere grew increasingly toxic.
Burnley even went to the extent of allowing children to sit on their bench to escape the crowd trouble.
The Hammers have launched a "full and thorough investigation" into the crowd trouble and an emergency meeting of London Stadium stakeholders has been called, the club said in a statement.
Manager David Moyes called for the club's fans to get behind the team.
He told Sky Sports: "We want the supporters behind us, but you can't cross the line and come onto the pitch.
"I don't think I've been at football games in my time where I've seen that. All I'll say to all of them is that we need them. We're fighting for points. We're all in it. The players know we did well today for long periods. We didn't do so well for a couple of minutes in the second half. We needed the supporters with us in a different way.
"All I ask for is that everybody sticks together and we try to get ourselves enough points to make sure we're a Premier League team (next season)."