One person has been taken to hospital serious head injuries and another six sustained minor injuries.
Officers were called to Prior Leaze Lane at 9.40am and were joined by ambulance crews.
A spokesman for Sussex Police said: "Police were called to Prior Leaze Lane, Hambrook at 9.40am on Thursday (23 April) to reports of an altercation. Officers attended with ambulance crews.
"One person was seriously injured and has been taken to hospital with a head injury.
"A further six people sustained minor injuries.
"An investigation to establish the circumstances surrounding this incident is underway. The group of people are believed to be known to one another and this is being treated as an isolated event."
Anyone who witnessed what happened is asked to report online or call 101, quoting Operation Goldwell.
The Observer apologises for the use of a picture in this story which may have unintentionally caused distress. Addresses shown are in no way linked to the incident. It has since been removed.