As many as 75 percent of children have seen images online which have 'disturbed' them, says the NSPCC.
The NSPCC said it was "alarmed" by the results of the poll on children's website www.there4me.com.
Zoe Hilton from the child protection charity said: "children are just a few clicks away from innocently stumbling across upsetting or even dangerous pictures and films such as adult sex scenes, violent dog fights, people self-harming and children being assaulted".
The charity has called for social networking and video hosting sites to be more pro-active in removing offensive material as soon as it is reported. The NSPCC also said PC manufacturers should be installing child protection software before the machines are sold.
"Currently computer manufacturers and retailers leave it to parents to find and install software that filters out material unsuitable for children," said Hilton
"Social networking sites must put more effort and resources into patrolling their sites for harmful and offensive material and ensure their public complaints systems are clearly marked, easy-to-use and child-friendly."