Video-sharing website YouTube has removed hundreds of pornographic videos which were uploaded in what is believed to be a planned attack.
The material was uploaded under names of famous teenage celebrities such as Hannah Montana and Jonas Brothers.
Many started with footage of children's videos before groups of adults performing graphic sex acts appeared on screen.
YouTube owner Google said it was aware and addressing the problem.
Disturbing videos
The BBC was made aware that scores of sexually explicit and inappropriate material was uploaded.
In one video titled Jonas Brother Live On Stage, a user posted a comment saying: "I'm 12 years old and what is this?"
http://newsimg.bbc.co.uk/shared/img/o.gifhttp://newsimg.bbc.co.uk/nol/shared/...t_quote_rb.gif This kind of raid showed how easy it is to upload porn to a website that millions of people browse on a daily basis http://newsimg.bbc.co.uk/nol/shared/...d_quote_rb.gif
"Flonty"
Under other uploaded videos, online users posted comments such as: "Take the tags off, you'll get us caught." Another said: "Your gonna kill us all!"
As the disturbing videos were being uploaded, many viewers added them to their favourites and rated them highly.
One user believed to have uploaded some of the pornographic material videos is Flonty, whose profile states that he is 21 and from Germany.
He told the BBC: "I did it because YouTube keeps deleting music. It was part of a 4Chan raid."
4Chan is a bulletin board focusing on Japanese manga and anime but some of its more extreme sections have been described on online messageboards as "sick" and "horrifying stuff".
"Anything and everything can, and usually does, happen here. We have our very own unique culture, and there is no group quite like us anywhere out there".
Community guidelines
When asked if he was concerned that children can freely watch such inappropriate material on YouTube, Flonty replied: "Children will find inappropriate material around the internet anyway.
"This kind of raid showed how easy it is to upload porn to a website that millions of people browse on a daily basis".
Google spokesman Scott Rubin told BBC News: "We are aware of the slew of pornographic videos that were uploaded.
"We are addressing them as we would any video that violates our community guidelines.
"In addition, any account we discover that has been set up specifically to attack YouTube will be disabled."
