Concert-goers at a Dutch festival have been treated to an unusual sight: a hot air balloon 40 metres high, shaped like a condom, drifting across the sky.

The balloon, with the words "Vrij Veilig" — Dutch for "Safe Sex" — emblazoned on it, was launched by the public health service at the festival in Lichtenvoordein.

The director of the health service, Laurent de Vries, said the festival was an ideal opportunity to reach young people.

"This is a playful way of asking for attention to the problem of sexually transmitted diseases, HIV and AIDS," he said.

More than 80,000 are expected to attend the three-day Zwarte Cross event - a combination motocross race and hard rock concert.

It took three months to design and construct the balloon in Bristol, England, and 10-15 minutes for workers from Virgin Balloon Flights Benelux to fill the balloon with hot air.

Organisers said they plan to submit the balloon to Guinness Book of World Records in the category of "world's biggest condom".