Emergency services were called to the Sporting Globe Bar and Grill Bendigo, in Central Victoria, about 9pm on Thursday, Victoria Police said.
The 41-year-old man was treated by paramedics before being taken to hospital in serious condition with suspected stab wounds.
Police believe he had been sleeping rough and was set upon by a group of youths following an altercation in nearby Bath Lane.
Detective Acting Sergeant Justin Ludeman said the attackers, believed to be teenagers, fled and left the injured man to search for help.
"Their behaviour is extremely cowardly," he told reporters on Friday.
"This male is just going about his everyday business and he appears to have just been set upon by these four males."
CCTV footage appears to show a group of youths wearing black balaclavas mouthing off at the victim.
An Ambulance Victoria spokesperson told AAP the man was taken to Bendigo Hospital in stable condition with upper body injuries.
The bar says staff are "deeply concerned" by the incident.
"An injured person made their way to The Sporting Globe Bendigo at approximately 9pm and collapsed in our sports bar," the venue said in a statement on Friday.
"Our staff acted immediately, contacting emergency services and initiating first aid."
Owner Leigh Stevens praised the efforts of his staff.
"The only thing I'd like to add is that I'm extremely proud of how my staff and patrons acted during this incident," he told AAP.
"They provided first aid until emergency responders arrived and should be commended."
Police are appealing for anyone with information to come forward.
Premier Jacinta Allan, who is also the MP for Bendigo East, expressed shock at the attack.
"That was an absolutely horrifying incident and my thoughts are with the victim and their family and loved ones," she said.
"I'm advised that police are investigating, and if there is anyone in the community who can assist police in that investigation I would encourage them to provide any information they may have."
Ms Allan, whose government has faced pressure over rising crime rates, denied police were powerless to deal with repeat offenders.
She pointed to strengthened knife seizure powers, a machete ban, tougher bail laws and higher sentences for youths who commit serious crimes.
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