More than 20% of children aged from eight to 17 have been bullied with images or videos online, according to a new report.
The report, carried out by the Safer Internet Centre, also found that 70% of young people have seen images and videos not suitable for their age.
The findings are part of the seventh Internet Safety Day campaign, which aims to inform young people about bullying and privacy online.
This year the focus is on images and videos, including the pressures they may put on young people about body image.
According to the study, which polled a sample of 1,500 children aged from eight to 17, young people take an average of 12 selfies before they are happy to post one online.
Some 43% said they were worried about how attractive the look in pictures posted online, while 45% of 13 to 17-year-olds said they have seen nude or partially nude photos of children they know being shared in school or in their community.
As part of Internet Safety Day, schools are showing films to encourage pupils to use images and videos safely.
Workshops and youth events will also be held around the country and more than 1,000 organisations are supporting the campaign, including Sky, Facebook, Twitter, Snapchat and the UK Government, as well as police services, charities and schools.
Edward Timpson, minister for vulnerable children and families, said: “The internet is a powerful tool that gives children and young people many fantastic opportunities – but protecting them from the risks they might face online or on their phones remains absolutely vital.”