The blaze broke out at Power Gymnasium on Holmes Street in Brunswick at 5.15am on Friday.
Flames and smoke engulfed the roof and shopfront, leaving the business severely damaged and ash scattered through the premises.
Investigators are working to determine the source of a blaze at a gym with alleged bikie links.
No one was hurt in the emergency.Â
Several media reports claim the gym has links to former Mongols bikie Sam Abdulrahim.
The professional kickboxer was shot four times in the chest in June 2022 when his Mercedes was ambushed as he left his cousin's funeral in Fawkner.
He survived the attack and managed to drive to a police station to seek help.
A post on the gym's Instagram page on Friday morning claimed the blaze was caused by an electrical fault at reception.
"It is with a deep heart that we have to advise all of our members due to unfortunate circumstances the gym will be closed for the next few days," the post said.
"Please do not feel threatened or intimidated. We look forward to welcoming you all back very soon."
View this post on Instagram A post shared by Power Gymnasium (@powergymnasium)
Victoria Police has not pinpointed the cause of the fire.
"An initial assessment of the extensively damaged building this morning was unable to determine the cause," a spokeswoman said.
"A Victoria Police arson chemist will attend the scene today to work out whether the fire was deliberately lit."
Abdulrahim has frequently promoted the gym on his own social media channels and posted videos of himself training there.
Anyone with information, CCTV or dashcam has been urged to come forward.
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