A dispersal order was put in place at Cosmeston Country Park over the weekend following reports of dangerous and anti-social behaviour.
The order was issued by South Wales Police to support the Vale of Glamorgan Council after people were seen jumping into the lake from height and swimming in the water.
It allowed police officers to direct individuals or groups to leave the area if their behaviour was likely to cause alarm or distress to members of the public. Police also carried out patrols at the park.
Rob Thomas, Chief Executive of the Vale of Glamorgan Council, said: “We have been in discussion with South Wales Police about a dangerous situation at Cosmeston Country Park.
“People have again been jumping into the lake from bridges and swimming in the water.
“Repeated requests to stop have been ignored, as have warnings about the risks of entering open water without lifeguards and safety precautions in place.
“That has left us with no option other than to ask the Police to take formal action to prevent serious injury or worse.
“Entering open water is extremely foolish and some of the behaviour witnessed beggars belief.
“The message is simple: Don’t enter the water at Cosmeston. Cold water shock can kill as can strong currents and hidden hazards.”
The council has been dealing with a range of anti-social and dangerous behaviour, including reports of people jumping into the water from Penarth Pier and Rhoose Point.
Fences installed to prevent access to the lake at Cosmeston for swimming were also deliberately damaged.
Elsewhere, a picnic bench at Porthkerry Park was destroyed by a barbecue, while a deliberate fire at a play area forced it to close.
The incidents came as South Wales Fire and Rescue Service dealt with multiple wildfires across the region.
The council has been cutting meadows to reduce the risk of fires, while a camera car has been patrolling Cosmeston in an effort to keep visitors safe.
Options are also being explored to install more permanent cameras at Cosmeston and Penarth Pier.
Council enforcement and community safety teams have increased patrols in some areas alongside neighbourhood policing teams.
Parents have been asked to remind young people about the dangers of entering open water and encourage them to swim only at designated bathing locations.
