Saul "Canelo" Alvarez and Caleb Plant traded blows on Tuesday as a press conference to hype their November showdown for the undisputed world super middleweight crown erupted into violence.
Unbeaten American Plant was left with a cut under his right eye following the fiery exchange at the Beverly Hilton hotel in Beverly Hills.
Mexican superstar Alvarez, widely regarded as the best pound-for-pound fighter in boxing, triggered the melee after coming face-to-face with Plant.
With both men appearing to trade expletives, Alvarez shoved Plant forcefully in the chest, sending the 29-year-old from Tennessee staggering across the stage.
Plant responded with a wild left hook - which Alvarez ducked comfortably before landing two counter-punches, causing a cut under Plant's right eye.
Security and members of both fighter's entourage both scrambled onto the stage to break up the fight.
The following press conference also threatened to erupt after Plant again addressed Alvarez with an expletive-laden tirade before accusing the Mexican of being a "drug cheat."
Alvarez tested positive for a banned steroid in 2018, forcing postponement of his fight with Gennady Golovkin.
"You're a drug cheat ... you're a cheater," Plant snapped.
"Don't make excuses before the fight," Alvarez responded angrily.
"Say whatever you say. I'm gonna see you on November 6. You're gonna see something special, and feel something special."
Alvarez later said the face-off turned violent after Plant insulted his mother.
"He can say whatever he wants to me, but not to my mum," Alvarez said. "He swung, I just pushed him."
Plant meanwhile insisted Alvarez triggered the brawl.
"All we was doing was staring at each other. He had something to say, I had something to say. He's gonna beat me, I'm gonna beat him," he said.
"And then he did what he did."
Alvarez will defend his WBA, WBC and WBO crowns against Plant, the IBF champion.
Alvarez, 56-1 with two drawn and 38 knockouts, seeks his eighth win in a row. Plant's record stands at 21-0 with 12 knockouts.