Not a crystal clear recording but it is a great document of the times. This gig took place at the Trinity community centre on this day in 1980. £1 entry.
Trinity centre is right by St. Pauls which saw race riots a couple of months earlier:
Tensions were still high. A couple of songs in and you can hear a crowd of about 50 raid the centre. (5:35)
In the comments for the video David Strut writes: "There was huge fight at the beginning of the Crass's set, About 50 Royalists wearing crash helmets and armed with home-made coshes stormed Trinity Hall and beat the fuck out of any punks that were unfortunately standing at the back before getting overpowered by the rest of the punks there and sent on their way."
The royalists referred to were probably made up of skinheads, National Front and others singing the anthem and a few other bits like SHAM 69's "Hersham Boys":
Then the bottles and glasses started flying.
The crowd manage to control things enough for CRASS to play the rest of the set, the recording ends on "They've Got a Bomb".