Tyson Fury's dad just went ballistic leading up to this weekend's big fight ... with John headbutting a member of Okeksandr Usyk's crew -- resulting in a bloody mess.
Fury and Usyk are slated to face off in Saudi Arabia on Saturday ... but the 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.