Police said late this afternoon (June 27) they have made 23 arrests in connection with a large unlicensed music event, described as a "rave", near Steyning.
Those detained are suspected of offences including drink and drug-driving, possession of drugs, and theft.
All 23 have been taken into custody.
A direction to leave has been issued to those at the site.
If this is not adhered to, and providing it is safe to do so, officers told the media that they will move to shut down the event.
Emergency services have also been met with hostility at the site, with one police officer being assaulted.
A second officer has also been injured in a collision with a vehicle being driven by someone leaving the event.
A significant number of people have travelled to the site from areas outside of Sussex, and enquiries were continuing to identify the organisers responsible for this unauthorised event so enforcement action can be taken against them.
A witness reported an estimated 5,000 people were present.
Officers remained at the scene by late afternoon, and pledged to continue dealing "robustly" with people found to be committing crime and behaving in an anti-social manner.
Those leaving the event were being stopped and checked, to ensure drivers are in a fit state to drive.
Police said disruption continued on the local road network, owing to a large number of badly parked vehicles.
Photographs and film from the scene showed scores of cars and vans abandoned on narrow country roads, in many cases blocking gates to farmers' fields.
A request to avoid the area still stood by 5pm.
Specialists in historical sites, of which there are many on the South Downs, commented on Sunday afternoon:
"There are burial mounds, dykes & enclosures spanning thousands of years.
"Our ancestors from Neolithic times, the Bronze Age, the Iron Age & the Romans all used this landscape.
"It is unknown at this time if any of these features may have been damaged.
"Archaeology, heritage, environment, wildlife and farming will undoubtedly have been affected.
"Trespass will damage and destroy habitat, while potentially causing irreversible damage & loss to the historical environment.
"Attendees also risk lives by letting livestock loose, impacting on farming, damaging crops, boundaries and gates, leaving litter & contaminating the environment."
Chief Superintendent Nick May said this morning:
“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.
“This is an ongoing matter and we thank the public for their patience and cooperation at this time.”
A social media account, claiming to follow 'rave' culture, had posted this footage on Sunday afternoon, claiming it to be from the unlicensed event at Steyning.
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