'Big Brother 15' -- Race War Explodes ... Aaryn Gries Mocks Black Houseguests

The race war inside the "Big Brother" house came to a head last night ... when racist Aaryn picked a fight with a black female contestant ... and MOCKED her by saying, "Whatchu gon do, gurl?"

She continued in an over-the-top stereotypical black voice ... saying, "Whatchu gon do gurrl ... class girl? Where's yo class?" (drama begins at the 2:00 mark)

It was disgusting and vile -- a callback to a white slavemaster talking to a slave on a plantation.

Eventually, Candice -- who's African American -- stood up for herself ... only for another female contestant, GinaMarie, to get in her face ... saying, "You want the black to come out?"

As things got even more intense, Howard -- who's also black -- grabbed Candice and begged her to stand down and not fight with the other racist houseguests.

FYI -- the fight started over a "prank" Aaryn pulled on Candice -- by flipping over her mattress.