The video-sharing site YouTube was loaded up with pornographic videos disguised as regular clips, many with kid-friendly tags like "Jonas Brothers," as part of an attack this week dubbed "Operation Porn Day."

Some explicit thumbnails were still expected to be visible on the site Friday, two days after the mass upload of pornographic videos by those who claimed responsibility for the attack, users of the 4chan website, a range of media outlets reported. The 4chan site is a popular online forum for sharing images, mainly manga, but also pornography.

The explicit videos uploaded to YouTube were typically disguised with about half a minute of non-pornographic content at the beginning.

Google-owned YouTube issued a statement Thursday saying it was aware of the issue and removed the videos as they were brought to its attention through its flagging system, "as we would any videos that violate our community guidelines.

"In addition, any account we discovered that was set up specifically to attack YouTube was also disabled."

According to YouTube's community guidelines page, videos flagged as inappropriate are not automatically taken down, but reviewed to determine whether they violate the site's terms of use.

Violations include contravening community guidelines that ban content such as pornography, graphic violence and copyrighted material.

Google spokesman Scott Rubin told the technology news website Ars Technica that the deleted videos were no longer viewable, but it could take a couple of days for video search results and thumbnail images to disappear from the site.

The Porn Day attack took place the same day YouTube announced on its official Canadian blog that 20 hours of video are now uploaded to YouTube every minute, up from six hours in mid-2007.

The 4chan site, started in 2003, has received media attention in the past for being the alleged birthplace of other internet phenomena, including pranks such as "Rickrolling." The 2008 prank involved posting links that appeared to direct the user to certain other websites, but actually brought up a music video of 1980s pop star Rick Astley.