A teenager, who cannot swim, was pushed into a canal when she refused to hand over her mobile phone to her attackers.
The six-strong gang surrounded the 18-year-old beside the River Kennet in Reading and threatened her with a knife.
When she refused to give them her phone, they slashed her hand and then pushed her in the river, before cycling off.
Fortunately two passers-by saw her struggling in the water between Caversham Bridge and Cemetery Junction and pulled her to safety.
The victim was left deeply shocked by the attack at 2330 BST on 9 September, the details of which have only now been released.
Police have praised the actions of the two men who saved her and a taxi driver who noticed her distressed state and took her home.
Detectives are now appealing for these men to come forward.
Detective Constable Pauline Foy said: "This was an appalling attack on a young girl who was walking home alone after a night out.
'Kind-hearted'
"Not only was she threatened by this group, she was then thrown in the river and left by them, she could not swim and her life was saved by the two heroic passers-by.
"I need to speak to these men as soon as possible and would like to reassure them that all calls will be treated in the strictest confidence.
"A kind-hearted taxi driver picked the girl up and took her home and I also need to speak to him urgently as well.
"The victim has been left very shocked and traumatised by what happened to her and she is being very brave and working with me to try and find those responsible," she said.