A 19-year-old has been beaten over the head with a metal pole during a violent armed robbery in south-west Melbourne.

Four people wearing masks forced their way into a fast food restaurant on Fitzgerald Road in Laverton around 3.20am on Tuesday.

The group threatened staff and demanded money from the register, police said.

During the altercation, a 19-year-old worker at the restaurant was hit with a metal pole, causing an open gash to his head.

He was later taken to hospital with non-life-threatening injuries.