The number of obese children and adolescents worldwide has jumped tenfold in the past 40 years, according to the World Health Organisation.
The WHO says the rise is accelerating in low and middle income countries, especially in Asia.
In Europe, girls in Malta and boys in Greece had the highest obesity rates, while girls and boys in Moldova had the lowest.
The organisation carried out the largest ever study based on height and weight measurements of 129 million people.
“Overweight in childhood and adolescence also causes social psychological problems for the children themselves, more stigmatism, more bullying, less optimal school performance,” said Leanne Riley, team leader of Noncommunicable Diseases Surveillance and Population-based Prevention at the WHO.
Nearly eight percent of boys and six percent of girls worldwide were obese in 2016, against less than one percent for both sexes in 1975.
Another 200 million children were overweight, but fell below the threshold for obesity.
Campaigners are calling for authorities to take action now.
“Countries will start at different places, perhaps in the schools, perhaps in the physical activity, perhaps in the public education and awareness and the regulatory and marketing, but all countries can tackle obesity through these six recommendations,” programme manager from the same department at the WHO.
The world health body says tax measures and tough restrictions on the marketing of junk food should be considered.
It wants clear food labels on salt, sugar and fat content, and has already recommended a 20 percent tax on sugary drinks.
Its study warns that if current trends continue, in five years there will be more obese children and teenagers and underweight ones.