Scores of youngsters are being warned they are risking their lives by jumping from Swansea's new Sail Bridge into the Rive Tawe.
A youngster prepares to jump from the Sail Bridge
Police and council officers say the craze is highly dangerous as the water below can be freezing despite the recent warm weather.
More than 10,000 people have signed a petition calling for improved safety measures at the nearby marina following the death of a 21-year-old man there earlier this year.
But despite the dangers there were at least a dozen youngsters making the 20ft leap into the river after school on Tuesday.
The water temperature is still very cold and that there have been deaths as a result of the shock under these circumstances
Swansea Council spokesman
They are not breaking any laws but police have been to the river to try to persuade them to stop.
A police spokesman said; "We would advise anyone against this dangerous behaviour.
"Sudden cold water immersion can lead to cramps, causing difficulties in the water.
"Lots of people have died in this way."
A spokesman for Swansea Council repeated the warning.
"We echo the views of the police that the water temperature is still very cold and that there have been deaths as a result of the shock under these circumstances."
More than a dozen youngsters were jumping off the bridge on Tuesday evening
The pedestrian bridge was built earlier this year as part of a £200m project to regenerate the marina and docks area.
But with soaring temperatures on the weekend it has become a magnet for thrill seeking youngsters.
Legal executive Craig Lucas died in the marina area in April after apparently falling into the water after a night out.
The 21-year-old's parents, Charlotte and Malcolm Lucas of Llanelli, have launched a petition calling for more railings at the waterside area.
However, youngsters jumping off the bridge after school on Tuesday evening said they did not see what the problem was.
One 14-year-old said: "I've been swimming here since I was nine.
"There are cold patches in the water but after a cycle or an ice cream on a day like this it is a good way to cool off.
"On Sunday there were lots of people jumping here - people we have never seen before."
His friend, who is 15, said: "There is not a lot to do in Swansea that is free and we are not causing any trouble.
"We were moved on from by the yacht club because they complained of the noise.
"I don't think we are doing anything wrong."