Football game going on then some gangbangers come and surrounds the game. Some of the audience comes down to the field to join the football players to fight against the gangbangers. That's all I can remember

(I haven't seen this movie) could it possibly be the Gridiron Gang?

Sorry that's not it. This movie I'm talking about is an old one. I remember seeing it as a kid in the late 80's early 90's.

Okay - (once again I haven't seen this movie) I think I found it, here is a bit of the plot I took from IMDB for a movie called The Wanderers:

*Focusing around a football game where the different gangs play with and against each other, then at its grand finale, come together in a mass of union to defend their honour and their turf. Nostalgic stuff and above all a Rock n' Roll retrospective on a grand musical era.*

Here is the link to that page: The Wanderers (1979) - Plot Summary
I hope this is it!

This might be it. I will take a look at it and get back to you. Thanks