Footage released by police shows a man following a woman off a bus, dragging her onto a driveway and attempting to rape her on a busy road.
It happened in Dagenham, east London, after the 22-year-old woman got off the EL2 bus on Wood Lane.
She was walking along Becontree Avenue when she was thrown to the floor before the attacker removed his jeans and exposed himself.
CCTV footage shows the woman fighting the assailant off with a punch to the stomach. As she shouted for help, she was able to attract the attention of a passing motorist.
As the vehicle reverses along the road to help, the attacker can be seen pausing before running off in the direction of Wood Lane. The motorist then called the police.
Police described the attacker as a man in his 30s with fair hair and a short brown beard.
He was wearing a grey top with a white shirt underneath and blue jeans.
Police are asking anyone who may have witnessed the incident on Monday at around 6.30pm to get in touch.
No arrests have been made.