Council bosses say Friday’s Park It event will still go ahead in Ribbleton Park, after travellers pitched up on three sites across Preston, including the green space off New Hall Lane and Blackpool Road.
The event, due to begin at 5pm, will move to the other side of the park, away from the caravans.
Police and Town Hall bosses received reports of caravans arriving in different locations in the city on Thursday.
Travellers were reported at Ribbleton Park and Trefoil Way off Bluebell Way, and also at the Port Way Park car park.
There are thought to be at least six caravans and a number of horses on Ribbleton Park, two caravans off Trefoil Way, and at least six at Port Way.
Police said they had been made aware, and had let the relevant authorities know.
Commuters have been told buses will run as normal from the park and ride car park at Port Way.
A statement from Preston Bus said: “Yesterday (Thursday) afternoon, a number of traveller caravans moved onto the Port Way Park and Ride car park.
“There are currently six caravans present, and access into or out of the site for cars and buses is still possible.
“For the rest of today (Friday), the site will operate as usual.
“Customers are advised to park well away from the caravans.
“The site will be manned all day to ensure the safety and security of our customers.
“Customers should be assured that we are doing all we can to move the caravans and avoid closing the site, but this can be a protracted legal process.”
A Preston Council spokesman said: “Officers from the council have visited the sites and ensured the welfare of everyone involved.
“We are now working with partners such as the police and Lancashire County Council and are in dialogue with the communities to determine what happens next.”
The news follows the arrival of travellers at a golf centre in Bispham on Wednesday, days after about 40 caravans arrived at Freeport Fleetwood.