EXCLUSIVE

Ex-UFC fighter Lavar Johnson allegedly beat the crap out of his girlfriend -- bashing her face in with a closed fist around a dozen times ... TMZ Sports has learned.

We've learned Johnson -- who also fought for Bellator and Strikeforce -- was arrested in Fresno, CA back in August after his girlfriend told cops the 6'3" giant went ballistic during a domestic dispute.

Among the allegations, Johnson's GF says he slammed her against the wall and ground, kneed her in the face and head multiple times and then struck her 11 more times in the face with a closed fist.

Johnson's GF says the fighter eventually backed off when he saw the blood streaming down her face.

When cops arrived, Johnson was gone -- but the girlfriend was clearly busted up ... with swelling across her forehead and nose and blood everywhere. She later needed staples to close one of the gashes.

Cops tracked down 38-year-old Johnson and arrested him on a slew of felony charges including assault with a deadly weapon (his fists).