Police were called by a member of public at 8pm on Monday, saying a group of travellers were trying to get in through the main gates off Liverpool Road.
When officers arrived on the site, the travellers moved on, but then were reported to have returned at 9pm.
This morning a large encampment could be seen on the playing fields.
It is not known if they are the same families who parked their caravans in Moor Park, Preston, the day before.
Police will be working with South Ribble Council to remove the travellers from the fields.