footage shows.
AJ then got ahold of Usyk's WBA and Ring Magazine belts, lofted them in the air, before chucking both out the ring.
It left the champion visibly confused as Joshua made his way to the other side of the ring.
Usyk's trainer Yurii Ivanovich Tkachenko then appears to make a comment to AJ, sparking an angry response.
The Londoner, 32, had to be held back as he was seen getting aggressive with Derek Chisora, 38, among those trying to help.
Eventually, AJ retuned to the ring and produced a bizarre post-fight speech.
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He said: “If you knew my story you’d understand the passion.
"I’m no amateur boxer from five years old, that was an elite prospect from a youth. I was going jail, I got bailed, then I trained my arse off.
"Because if I got sentenced I wanted to be able to fight. I’m stealing this Usyk, sorry.
“It’s because the passion to get into this shit. This guy, to beat me, tonight maybe I could’ve done better.
"It shows the levels of hard work he must’ve had to put in. Please give him a round of applause as our heavyweight champion of the world.
"I’m not a 12-round fight, look at me, I’m a new breed of heavyweight.
“Heavyweights like Mike Tyson, Sonny Liston, Jack Dempsey, ‘You don’t throw combinations like Rocky Marciano!’
"Because I ain’t 14 stone, that’s why, I’m 18 stone, that’s why, I’m heavy."
"It’s hard work, this guy is a phenomenal talent, we’re going to cheer for him three times.
I’ll be the first to admit, I let my self down. I acted out of pure passion and emotion and when not controlled it ain’t great.
Anthony Joshua on Twitter
"How many belts you got now bro, five? Hooray. I was studying Ukraine and their champions from your amazing country.
“I’ve never been there, but at the same time, what’s happening there, I don’t know what’s happening. It’s not nice.
"I’ve seen it in Lomachenko’s second fight. There were issues, civil war. Vitali Klitschko against Danny Williams, civil unrest.
"Usyk, as a champion, please raise your hand, under those circumstances you became a champion. Champ, champ.”
Joshua's emotional outburst was slammed by Carl Froch, 45, and even training partner Frazer Clarke, 30, but Eddie Hearn leapt to AJ's defence.
Hearn said: “The reaction from AJ - he’s a human, so much pressure. He exploded, he was devastated."
Joshua himself apologised on Twitter and congratulated unbeaten Usyk, 35, once more on his win.
He posted: "I wish @usykaa continued success in your quest for greatness. You are a class act champ.
"Yesterday I had to mentally take myself into a dark place to compete for the championship belts!
"I had two fights, one with Usyk and one with my emotions and both got the better of me.
"I’ll be the first to admit, I let myself down. I acted out of pure passion and emotion and when not controlled it ain’t great.
"I love this sport so so much and I’ll be better from this point on. Respect."