Saul ‘Canelo’ Alvarez and Caleb Plant threw punches when their first face-off descended into a wild brawl on Tuesday night.
The atmosphere was already tense when Canelo and Plant when eyeball to eyeball, both muttering insults to each other.
Canelo aggressively shoved Plant backwards. Plant then threw the first punch and Canelo responded by throwing a shot of his own until they were separated by security.
Plant was cut underneath his right eye by the melee.
Canelo and Plant will decide the undisputed super-middleweight championship on November 6 in Las Vegas.
Mexico’s Canelo holds the WBA, WBC and WBO belts – since December, he has already ended the undefeated reigns of British duo Callum Smith and Billy Joe Saunders.
America’s Plant holds the IBF title, the only remaining belt outside of Canelo’s grasp, and is unbeaten in 21 fights.
Canelo is a four-weight world champion widely recognised as the No 1 fighter in the world.