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 Monday.
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 .