A man has serious burns after a large fire broke out in a busy northern Sydney shopping plaza.

The fire was started after a gas main caught alight on Monday afternoon at the front of a shop in Lane Cove, Ian Krimmer from NSW Fire and Rescue said.

A man in his early 20s suffered serious burns to his lower body and was taken to Royal North Shore Hospital, an ambulance spokesman told AAP.

Six fire crews are battling the blaze and working to move people out of nearby shops.

There are concerns the fire could spread to the adjoining shops.