Residents claim a gate was left open allowing scores of travellers to set up camp on Preston Park in Brighton.
About 30 vans and caravans arrived at the park on Wednesday evening.
John Brown, who lives close to the park, said an entrance was left open by the council after the circus left the site on Sunday evening making so the park easy for travellers to enter.
Mr Brown said: “I don’t understand why the council just left the entrance open for them to get in. “There are about 30 in there and there
have been more turning up, driving straight across the park.
“The entrance has been open since the circus left on Sunday. They turned up before – about four or five months ago – and there were 30 or 40 of them then. This time they’re not just parking up the sides of the park, they’re all over the park. “I just find it annoying the council doesn’t seal the place off.
“I’ve phoned the council and they’ve said they’re sending someone down with the police.”
A council spokeswoman confirmed that council officers had carried out a site visit following the report and they were now taking steps to deal with the situation.
She said: “We have visited this site with the police and we are currently undertaking a community impact assessment to determine the most appropriate course of action.
“Preston Park isn’t protected by bollards and is completely open on the west side on to Preston Road.
“The travellers entered the park at various points including driving over the grass at Preston Road and through the Preston Drove entrance.”
A police spokesman said: “At 9.56pm on May 21 police received a report that more than 30 travellers’ caravans, estimated, were moving onto land at Preston Park, Preston Park Avenue, Brighton.”
He was unable to provide any more details.