As Mayweather's Rolls-Royce was leaving the SSE Arena in London on Friday, dozens of McGregor fans swarmed Floyd's ride ... flipping him off and shouting insults at the car.

Security tried like hell to keep the hooligans away ... literally ripping people from the car ... but during the process, one guy appeared to get rocked by the front of the Rolls.

You can't actually see the collision -- but you can hear it -- and you can see the security guy go flying down the street. He appears to be okay.