Officers are appealing for information after a group of teenagers attempted to ignite fuel on an Asda forecourt and punched a woman who tried to stop them.
A care home nurse who intervened was attacked by one of the party at about 21:00 BST on 2 April at the petrol station in Bridgwater, Somerset.
The woman in her 40s had her arm twisted behind her back and was punched several times in the face by a girl.
Avon and Somerset Police said footage of the act was then posted on Snapchat.
The assault happened when a man challenged the group of teenagers.
It was after a boy, aged about 15, poured fuel out of the pump and attempted to ignite it with a lighter, police said.
The force is urging anyone who saw what happened or who filmed it to get in touch.