Nicki Minaj fans are clearly passionate ... both about her music and standing their ground, it seems -- 'cause a group of her supporters got into a fight during her tour's opening performance!

The Barbz brawl went down Friday night in Oakland ... the first night of her much-anticipated "Pink Friday 2 World Tour" -- and it definitely started the whole concert run off with a bang.

Just check out the clip ... a couple fans dressed in black are exchanging punches in one section -- with someone in an all-pink ensemble and dyed pink hair hopping into the scuffle.

The person in pink getting shoved back seems to set one of the angry fans off even more ... and they jump forward to start raining blows down on the other combatants.

Unfortunately for them, their opponents quickly put 'em in a headlock and start hitting them over and over while being held back against the seats.

Eventually, some security guards come over and break up the fight, but members of the crowd can be heard seemingly questioning the person in pink's gender -- overall, a pretty ugly scene.

The whole thing marred what was otherwise a very impressive show ... with Nicki breaking out all her classic hits including "Starships" which she said a few months ago she was done performing.