A motorcyclist is fighting for life after a serious collision in Melbourne's inner north.

The busy intersection of Brunswick Street and Alexandra Parade on the border of Fitzroy and Fitzroy North was closed after a Mercedes Benz collided with a motorcycle at about 10.45am on Friday.

Ambulance Victoria said the motorcyclist was rushed to the nearby St Vincent's Hospital with "serious injuries to his head".

Police said the 41-year-old male rider from Ringwood had life-threatening injuries while the 61-year-old male driver was uninjured.

The cause of the crash is being investigated and police are appealing for witnesses or dash-cam footage.