Croatian boxer and light heavyweight Loncar launched a brutal assault on referee Maciej Dziurgot, who decided to end the 81-kilogram bout in favour of Lithuania's Algirdas Baniulis at the European Youth Boxing Championships on Monday.
Dziurgot waved away the fight and prepared to declare Baniulis the winner before Loncar floored the Polish referee and landed several punches to the body.
The unprovoked assault was only stopped when Loncar was dragged from the ring by his legs.
Dziurgot was taken to hospital with "serious injuries", while police took Loncar into custody.
And the national boxing federation have imposed a lifetime suspension and issued an apology.
"This is the worst thing that could happen to us," said Croatian Boxing Federation secretary general Marko Marovic.
"This is a disgrace to Zagreb and Croatia and cause the sport of boxing major damage.
"All the hard work that we put into organising this championship and raising the image of Croatian boxing has been destroyed in a few seconds."