A 17-year-old girl was walking home from college at around 7pm on Tuesday (October 31) in the area of Churchill Gardens in Westminster.
As she stopped to cross the road, a man jumped out of a vehicle and grabbed both of her wrists before putting his hands on her waist in what police say was an apparent attempt to pull the victim into the vehicle.
The male then proceeded to put his mouth on her cheek when a passer-by intervened, and the man got into his car and drove away from the scene.
DC James Kapp, of Westminster CID, said: "This incident left the victim extremely shocked and upset, and we need to identify the suspect as soon as possible.
"We would particularly like to speak to the member of the public who intervened, and anyone else who may have witnessed this incident."
Police describe the suspect as a male of middle eastern appearance and aged around 30.
He is approximately 6ft tall, of large build and was wearing a dark hooded jumper, dark trousers and had a full dark-coloured beard.
The vehicle he got into is described as a small red car.
Anyone with information relating to the incident should call Westminster CID on 101. Alternatively contact Crimestoppers via 0800 555 111, or visit crimestoppers-uk.org.
Keep up to date with the latest news in west London via the free getwestlondon app.
You can set up your app to see all the latest news and events from your area, plus receive push notifications for breaking news.