The footage shows a group of teenagers confronting a woman behind the till at the fast food store in Chelmsley Wood, Birmingham.
One of the group picks up an object and throws it at the worker, who retaliates by throwing something back.
“Are you stupid, are you f***ing stupid?” a teen screams as the employee continues to hurl objects at them.
Suddenly, one of the group jumps behind the counter and shoves the worker.
A colleague rushes to her aid but the girl flings several punches at their head.
The scrap continues behind a counter – with those involved this time grappling on the ground.
Thousands of people have seen the footage since it was posted over the weekend.
The incident happened on Saturday night (April 27) and police are now investigating reports of an assault.
“It is believed a group of teenagers became verbally abusive to a member of staff, before jumping behind the counter, where one of the girls then kicked, punched and bit the member of staff,” a West Midlands Police spokesperson told Daily Star Online.
“The 21-year-old woman was taken to hospital to be treated for her injuries, which fortunately aren’t thought to be serious.
“Enquiries remain ongoing and anyone with information is asked to contact us, quoting crime reference number 20SH/100743L/19 via Live Chat at west-midlands.police.uk between 8am and midnight, call 101 anytime or contact Crimestoppers anonymously on 0800 555 111.”
A KFC spokesperson said: “We have a zero-tolerance policy when it comes to violence against our staff and were shocked to hear about what happened.
“We’ll be helping the local police with their investigation and supporting the team members who were affected.”