Youths seen clambering on the roof of Llangollen Pavilion was just part of a series of break-ins at the home of the international eisteddfod earlier this week, according to the company which runs the building.
Now Denbighshire Leisure Ltd says it is working closely with the police to catch those responsible who will be prosecuted, it warns.
The company says in a statement: "Groups of youths have been seen climbing onto the Llangollen Pavilion roof, as well as vandalising indoor and outdoor areas of the events venue.
"Denbighshire Leisure Ltd, who operate Llangollen Pavilion on behalf of Denbighshire County Council, is working with North Wales Police to catch those involved amid safety concerns and following the number of break-ins at the site this week.
"The company is calling on the local community to be vigilant and encourage youngsters to respect their local town.
"Incidents include a group of youths gaining access to the building, setting off the security alarms, vandalising bollards and lighting, and breaking areas of the tented arena. One group of teens has been filmed climbing on top of the pavilion."
Jamie Groves, Managing Director of Denbighshire Leisure Ltd, said: “This is incredibly dangerous and we would advise anyone to think twice before trying to gain access to roofs of any public building.
"This will result in prosecution and could result in serious injury. The emergency services are under enough pressure at the moment and this is mindless behaviour that will end in tragedy.
“We will not tolerate vandalism at any of our properties and we are working closely with North Wales Police to identify the individuals involved.
"The Llangollen Pavilion is an iconic events venue that brings thousands of people to the county every year. We have CCTV footage and this will be provided to the police to investigate further.
"Llangollen Pavilion is the home to the International Eisteddfod Music Festival that brings thousands of visitors to the town and county every year.
"Anyone with any information is encouraged to share this with North Wales Police."