Three out of four children have seen images on the internet that disturbed them, an NSPCC poll suggests.
The charity is renewing its call for computer manufacturers and retailers to install security to stop children finding violent or sexual content.
The NSPCC, which polled visitors to its children's website There4me.com, said it was "alarmed" by the accessibility of potentially disturbing material.
Some 377 of 497 votes cast claimed to have been disturbed by internet images.
One child posted a comment on a There4me message board saying: "I've seen violent images I didn't search for. I was freaked out."
Another said his eight-year-old sister's search for "pictures of animals" generated pornography adverts.
The NSPCC wants social networking and video hosting sites to remove offensive material within hours of finding it.
Policy adviser Zoe Hilton said the NSPCC was "alarmed" by how easy it was for children to access "disturbing internet material".
She 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."
Ms Hilton said that every child should be using a computer with child protection software.
"High-security parental controls installed in their computers would help shield them.
"Currently computer manufacturers and retailers leave it to parents to find and install software that filters out material unsuitable for children. This can be a complicated process for customers."
The charity wants retailers to ensure the software is installed before selling computers, and also manufacturers to start building such controls into their products.
She added: "Social networking sites must also 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.
"We would also recommend they give information on their sites about sources of help and advice, such as Childline, for children who have been affected by what they have seen."
No responsibility can be taken for the content of external Internet sites.
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