Boxing: Paul Gallen and John Hopoate clash at a fiery weigh-in ahead of heavyweight bout

Cronulla Sharks captain Paul Gallen grabbed John Hopoate by the throat, as tensions boiled over during their weigh-in for their charity boxing bout in Sydney.

After both fighters weighed in for their charity bout on Thursday, the pair went face-to-face and their heads were touching, when Gallen grabbed Hopoate, who responded by swiping at the NRL veteran.

A push and shove followed between the pair, knocking down sponsorship signage, before their respective teams separated them.

Gallen told 7 News he would knock out his opponent.

"He thinks he's so intimidating and tough. I'm sick of it," said Gallen.

"I'm going to be happy to get in there tomorrow night and shut him up once and for all.

"I think I will knock him out."

Before the fight, Gallen, who is undefeated at 8-0 (4KO) in his boxing career, said he'd retire from professional boxing if he lost. Hopoate has a 12-5 boxing record with 11 knockout victories.

On the scales, Hopoate was the heavier than the two, tipping the scales at 112kg, with Gallen at 101.5kg.

Gallen has previously called out All Blacks star Sonny Bill Williams numerous times as he is desperate to fight the dual-code international.

The pair have both bet $50,000 from their purses over who will win the fight. 

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Boxing: Paul Gallen and John Hopoate clash at a fiery weigh-in ahead of heavyweight bout