Check out this footage that we obtained, which we're told was taken very early Saturday morning at the Skate Rock City roller rink in Vegas ... where CB was celebrating his 34th birthday among a lot of celeb pals, including Usher -- with whom he's incredibly close.

While Usher and co. sang to Chris earlier in the evening when everything was fine, things unfortunately got ugly in the wee hours of the morning ... when eyewitnesses tell us Chris was attempting to talk to Teyana Taylor, who was also in attendance and sitting on a bench just outside of the rink. We're told, for whatever reason, Teyana was ignoring him.

Our sources tell us this pissed Chris off, and he started to yell at her ... and that's when Usher intervened and tried to calm him down. You can see that in our video here.

We're told Chris wasn't having it, cursing at Usher and Teyana -- but then, mostly focusing his anger directly at Usher. Eventually, our sources say Chris told his crew they were leaving and they exited the facility. We're told Usher took off his skates and went after them.

Our eyewitness -- who says they were present for this -- tells us they saw Usher go behind a string of charter buses that were parked in the lot ... where Chris and his crew were at. We're told a short time later, Usher emerged with what appeared to be a bloody nose.

Unclear if he went to the hospital, or if a police report was filed. We've reached out to Vegas Metro -- but haven't heard of anything even remotely resembling this being called in.

Meanwhile ... Usher, Teyana and Chris have all been radio silent this morning -- with none of them having said a thing yet to confirm or deny a report initially put out by Hollywood Unlocked, which broke the news of a fight between the two R&B superstars/friends.