It's euphemistically called "crowd trouble"

An anguished supporter on BBC RADIO FIVE LIVE online just described tonight's HOOLIGAN BATTLE at the WEST HAM-MILLWALL derby in the CARLING CUP as a troubling trip back in time, as football violence proved it has not been completely eradicated from the game.
A 44-year-old man was stabbed as hundreds of people clashed outside West Ham's Upton Park ground.
News reports tell of thugs chipping bricks off buildings to hurl at other supporters and police.
West Ham defeated Millwall, 3-1, and Hammers' supporters prompted stoppages of play when they invaded the pitch.
I was sickened to hear of it.