The victim ran off to a nearby house for help, while the suspect is said to have fled the scene in the direction of Bird in Bush Road.
When police arrived at the scene they searched the area but failed to find the suspect.
The suspect is described as being southeast Asian in appearance, possibly Chinese, approximately 40-years-old, of strong build, about 5ft 8ins, with short grey hair. He was wearing a blue polo t-shirt, light blue jeans and was holding a brown briefcase.
Detectives from Southwark CID are investigating and are appealing for information and witnesses to come forward.
Detective Constable Alison Hall, the investigating officer from Southwark CID, said: “The victim was understandably left very shocked and distressed by this incident. No-one should be subjected to this, especially not a child.