The fan in question was a Royal Antwerp supporter, and the keeper was Graham Stack, currently on loan to Arsenal's Belgian "(allegedly) work-permit avoiding" sister club Beveren.

Beveren were in the process of beating Antwerp 3-1, and after the late 3rd Beveren goal some of the opposition fans managed to get on the pitch....

"Out of nowhere, I saw all these fans running at me," says Stack.

"One bloke approached me and I told him to get off the pitch. I had to push him away from me for my own protection. Then I could see someone else running at me and it was obvious he was not going to stop. I swung at him out of self-defence."

"I didn't him that hard but he must have been drinking because he went straight to the floor. The next thing I knew, there were riot police everywhere. They came on with their shields and dogs to try to calm things down because it was a bit mad."

"Thankfully, the game was ended at that point and we were all taken off the pitch."

According to the Sun, Stack had been subject to quite a barrage of missiles during the game, including a knife! (found in the goalmouth after the game so maybe it was just part of Stack's self-defence kit).

Rupe adds...
Many thanks to Ray for reminding us that Arsenal are not the only Premiership club to have a relationship with a Belgian club. Royal Antwerp happen to be the feeder club for Man Utd!