A group of about 15 traveller vehicles have moved on to St Helen's Park next to Hangleton Way in Hangleton.
Police were on the scene as they tried to set up at 7pm this evening.
Officers on the scene believe the group had moved from Southwick Green, where the same number had resided.
Lee Wares, a Conservative candidate for Patcham ward on Brighton and Hove City Council, was meeting up with two local ward councillors when he took these pictures of the incursion.
Councillor Dawn Barnett, who represents Hangleton and Knoll ward on the council, said: "It is not good.
"The police say there is nothing they can do.
"There are a lot of elderly residents around here and they are going to feel vulnerable."
Coun Barnett had concerns about how the park would fare if travellers stay there.
She believes it is consecrated ground dating from 1086 when St Helen's Church, at the top of the park, was built.