If my memory serves me right it started getting nasty in the early to mid 60s, we had manc mates who we used to meet both at home and away, and after the match have a good night out.
One Saturday,in a manc pub(cant remember the name) which was a popular meeting place for both sets of fans, it kicked off big style, and the pub got wrecked, furniture smashed, windows and fittings smashed, a real wild west saloon type brawl, after that everything just got worse.
One funny thing during the brawl was,when I squared up to this manc I realised it was one of my manc mates and went for someone else, leaving him alone,likewise him to me also.