Fury and Usyk are slated to face off in Saudi Arabia on Saturday ... but the Gypsy King's pops couldn't leave the violence to the two boxers when their camps crossed paths on Monday.

The wild altercation broke out minutes ago ... with "Fury" and "Usyk" chants ringing throughout the room as John approached the Ukrainian fighter's group.

John was clearly unfazed about being outnumbered ... locking horns with someone in the front of the pack.

The two sides got broken up and things died down momentarily ... but when one dude started yelling "Usyk" in John's face again, he used his noggin as a weapon.

John was quickly dragged from the scene ... before turning around and yelling, "Show some respect" -- revealing his bloody forehead in the process.

John was asked about what exactly unfolded shortly after the incident ... and he explained "some little idiot" invaded his personal space, so he had to take action.

But, once cooler heads prevailed (and stopped leaking), John expressed his remorse.

"Sincere apologies to everybody involved," John said, according to Michael Benson. "It's just the way we are. Emotions and tensions are running high. He was a very disrespectful fella."

"If you come close in a fighting man's space, you're gonna cop for something. What matters to me is respect for my son and he wasn't showing any of it."

As for the blood, John said it doesn't bother him one bit -- after all, the Furys are fighters.