Residents near Reinwood recreation ground in Huddersfield, West Yorkshire, were furious when several caravans arrived on the green space. Councillors have also told of their frustration.
Cahal Burke, a Lib Dem councillor at Kirklees Council, said work had been completed at the recreation ground earlier this month but the barrier had been left unlocked.
It's believed residents had even contacted the authority, which Labour lost to no overall control last year.
Speaking to Yorkshire Live, Cllr Burke added: "It's extremely frustrating that travellers have gained access to Reinwood recreation ground through a barrier that was left unlocked.
"This is despite the fact that a resident contacted parks on 14 July to inform them that the barrier was unlocked.
"Work has been taking place near the entrance and I understand a third party contractor has failed to lock the barrier.
"There are three caravans on site and a number of vehicles. West Yorkshire Police's Neighbourhood Policing Team are aware and have been on site, a welfare check has also taken place.
"It is understood that the travellers will be on the recreation ground for a week however that could change and more could arrive.
"The corporate landlord is aware and the usual legal procedure for this type of encampment will be followed."
Kirklees Council, which meets in Huddersfield, has been approached for comment.
It comes after hundreds of residents in a quiet rural village blasted plans for a new traveller site nearby.
A council had considered a proposal for five authorised traveller pitches on land near Middleton, Northamptonshire.
However, residents in the village argued the development would encourage travellers from neighbouring areas to pitch on unapproved land.