The incident happened in Wrythe Lane near Westminster Road just before 9.50pm yesterday (August 19).
Officers and London Ambulance Service crews attended and found a man injured.
Wrythe Lane Carshalton crime scene after man injured: Live
He was treated at the scene before being taken to hospital where his condition was assessed as non life threatening.
No arrests have yet been made.
It is unknown which fast-food restaurant the incident took place at.
A spokesperson for Metropolitan Police said: “Police were called at 9.44pm on Tuesday, 19 August following reports of an assault in Sutton.
“Officers called the assistance of London Ambulance Service inside a fast-food chain on Wrythe Lane, Carshalton where a man had been injured after allegedly being assaulted by a group of men.
“His injuries have been confirmed as neither life-threatening nor life-changing.
“No arrests have as yet been made.
“Enquiries continue.
“Please call police on 101 with the reference 7990/19Aug if you have any information.
“You can also call Crimestoppers anonymously on 0800 555 111 or visit crimestoppers-uk.org.”
