Errol Spence and Terence Crawford ain't the only boxers fighting in Las Vegas ... Caleb Plant and Jermall Charlo got into it on Friday as well -- and the altercation between the two stars was wild.
You can see in video shot at T-Mobile Arena ... Plant and Charlo were backstage at the Crawford vs. Spence weigh-ins, when Plant charged at Charlo and slapped him right in the face.
Plant's hand connected so flush with Charlo's cheek, it actually made a loud sound that forced a gasp out of at least one bystander.