An attacker grabbed a woman as she walked home from an evening out and only ran away when the woman screamed.
Police are appealing for anyone that witnessed the attack in Strawberry Lane, Carshalton, between 11.15pm and 11.45pm on Saturday to come forward.
The woman, aged 44, had been at a friend's house in nearby West Street with her husband and two children. Her husband and children left while the woman stayed behind for several minutes to talk.
She then left and walked home alone from West Street into Nightingale Road where she briefly spoke to a man with a dog.
The woman then walked into Strawberry Lane and had gone past the junction of Devena End and the Causeway when a man approached her from behind and grabbed her, putting both his hands over her mouth.
As they struggled they both fell to the ground and the woman screamed. A neighbour ran out to see what was happening and the woman's husband heard the scream and quickly returned.
The attacker was a man of skinny build, 5ft 10in tall and was wearing a faded navy blue hooded top.
The woman said he had dark-skinned hands and long fingers.
Officers say they are puzzled by the attack as the man did not try to take the victim's bag and did not ask for money or say anything during the incident.
PC Matt Collins from Sutton CID said: "We believe the man may live locally as he quickly disappeared from the scene and seems to know the local area."
Anyone with information can call police on 101 or the charity Crimestoppers on 0800 555 111.