Three out of four children have seen internet images that disturbed them, according to a poll published today.
One child posted a comment on the message board of NSPCC children's website There4me.com saying: "I've seen violent images I didn't search for. I was freaked out."
Another youngster said his eight-year-old sister typed in 'pictures of animals' and ended up with pornography adverts.
The charity said it was renewing its call for all new computers to carry high security "blocking" software to stop children accidentally finding violent or sexually explicit pictures, films and pop-ups.
It also wants social networking and video hosting sites to remove all offensive and harmful material within hours of finding it.
Policy adviser Zoe Hilton said: "We are alarmed by how easy it is for children to access disturbing internet material.
"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.
"High security parental controls installed in their computers would help shield them.
"Currently computer manufacturers and retailers leave it to parents to find and instal software that filters out material unsuitable for children.
"This can be a complicated process for customers.
"Every child should be using a computer with child protection software. We want manufacturers to build this into every computer and set it to a high level of security.
"We also want retailers to ensure the computers have this software before it is sold to parents.
"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.
"Child protection technology cannot replace a parent's care, but it can be an important tool.
"Parents must also educate their children about staying safe online and show them how to report anything they see on the internet that upsets or worries them."