Homicide detectives are hunting for a killer on the run after a man was shot dead in front of shocked witnesses outside a factory complex in Melbourne's north this morning.

The man, believed to be in his 30s, was shot at least once in the face about 6.20am at Merri Concourse, Campbellfield.

He died at the scene.

The scene at Campbellfield. (Andrew Nelson, 9NEWS)

Police said the shooting followed an argument between the shooter and the victim, who was an associate of a group of people inside one of the nearby factories.

The street was put into lock down as the police helicopter searched the area for the killer on the run.

Detectives escorted several shaken witnesses for questioning, but none are being considered as suspects.

Forensic police and ballistics experts spent today examining the factory for clues, but there has been no sign of the murder weapon.

There is no clear motive for the killing and police have ruled out early reports of links to outlaw motorcycle gangs.