Pedestrian hit on Sydney's Princes Highway

A man has suffered a serious head injury after being hit and trapped under a vehicle in Sydney's south.

Emergency services were called to the Princes Highway in Arncliffe shortly before 1pm on Monday, where the man was trapped "for a period of time", a NSW Ambulance spokesman told AAP.

The man has since been transferred to hospital but all southbound lanes and one northbound lane of the highway are closed, the Transport Management Centre said in a statement.