Travellers have set up camp at Hill Barn Recreation Ground in Worthing.
About 16 caravans gained access to the site, after gates in Hill Barn Lane, opposite the old Aviva site, were broken through.
Herald reader Tom Barber took this photograph showing the damage to the gates, and police in attendance.
A spokeswoman for Worthing Borough Council said: “Travellers with approximately 16 caravans have illegally moved on to Hill Barn Recreation Ground on Hill Barn Lane by breaking down the perimeter fence.
“Legal services have been asked to commence proceedings to have the travellers removed from this site as soon as possible. It is not known at present where the travellers have come from but the number of travellers at Bost Hill in Findon Valley has dwindled to just six caravans overnight.”