Former heavyweight boxer Julius Francis -- who famously fought Mike Tyson in 2000 -- showed he's still got thunder in his hands ... knocking out a man cold while working a security gig on Saturday.
The ferocious punch went down outside of BOXPARK Wembley in the U.K. ... after a man appeared to get into a confrontation with security guards for an unknown reason.
In video of the incident, shot by a bystander, you can see the man in the du-rag was clearly not pleased with Francis and other guards ... screaming at them, and seeming to shove some of them as well.
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As the guards were trying to get the man away from the area, the guy stepped toward Francis -- who clearly didn't like that.
Video shows Francis hit the man square in the jaw with a quick right hook -- sending him straight to the ground. The man appeared to be out cold for several moments.
No word on if the guy suffered any significant injuries.
Local law enforcement, meanwhile, said in a statement they are investigating it all.
"Police are aware of footage circulating on social media showing an incident involving security staff and a member of the public outside Boxpark in Wembley."
"An investigation into the circumstances, including to establish the identity and welfare of those involved, is underway."
They added no arrests have been made at this time.
Francis fought professionally 48 times in the 1990s and early 2000s -- famously losing to Tyson in a January 2000 bout in Manchester.
The 57-year-old former pugilist -- who ended his career with a 23-24-1 record -- also famously fought Vitali Klitschko in 1998, losing by TKO.