A 41-year-old man is fighting for life after being brutally attacked with a hammer in “one of the worst” attacks police have seen.

The victim was walking along Hoddle Street in Collingwood around 4.30am when two men attacked him from behind, pushing him to the ground before instigating a “prolonged” attack with a hammer

Two men, a 19-year-old Fitzroy North man along with an 18-year-old Fitzroy man have been arrested after fleeing the scene.

The scene of a "brutal" assault in Collingwood. (9news)

Senior Sergeant Mia Patterson said the brazen attack in a public place was “absolutely brutal” and left the victim unconscious and suffering “graphic injuries”.

Officers are unsure whether the parties were known to each other at this point and are yet to talk to the victim after he was rushed to the Alfred Hospital for emergency surgery.

The exact circumstances surrounding the incident are yet to be established and the investigation is ongoing.

Police have arrested two men who are assisting officers with their investigation. (9news)