Multiple road closures are in place and people are asked to avoid the area 'if possible'.
"We are aware of and responding to a large unlicensed music event in the Steyning area this morning," a police spokesperson said.
"Anyone planning on attending the event is warned not to travel to the site – officers will be taking action against those found doing so."
Chief Superintendent Nick May said the incident remains ongoing, adding: "We thank the public for their patience and cooperation at this time.
"Our priority is ensuring the safety of the community and everyone in the vicinity.
“We have a significant number of officers at the scene working to bring the event to a safe and timely conclusion."
According to traffic reports, Bostal Road is closed both ways between Newham Lane and Clays Hill. This is affecting traffic between Sompting and Steyning.