The internet hacktivist group Anonymous says it temporarily took down more than 40 child pornography sites on a hidden network and posted a list of more than 1,500 of the sites' usernames online.

Earlier in October, "Operation Darknet" used denial-of-service attacks to disable a server called Freedom Hosting that it identified as "the host of the largest collection of child pornography on the internet," Anonymous said in a statement posted on PasteBin, a text storage site used mainly by programmers.

A particularly large site called "Lolita City," that contained more than 100 gigabytes of child pornography, the group added. Denial-of-service attacks flood websites with traffic, making them unavailable. In this case, the Freedom Hosting sites were unavailable for 30 hours, Anonymous said.

The group then demanded that all child pornography be removed from the sites.

The sites, located on a hidden "darknet" on an anonymous network called Tor, refused and managed to restore their services, but Anonymous has been targeting them with other tools since then, the group said.

"We will continue to not only crash Freedom Hosting's server, but any other server we find to contain, promote, or support child pornography," it added in a statement.

The Tor network and a related site, I2P, were originally designed for and used by activists in countries with authoritarian governments, such as China and Iran.

'Unfortunately, a potentially benevolent resource has been corrupted by these sick and sadistic abominations of the world," Anonymous said in a statement posted on YouTube on Oct. 17.

Some internet users praised Anonymous's campaign as "awesome" and a "great job" in messages posted on YouTube, Twitter and other online forums.

Campaign may put more kids at risk: security blogger

However, the group also faces criticism for the attacks.

Graham Cluley, Vancouver-based senior technology consultant at internet security company Sophos, wrote on the company's Naked Security blog Monday that Anonymous's " intentions may have been good, but take-downs of illegal websites and sharing networks should be done by the authorities, not internet vigilantes."

He suggested that such "amateur" attacks could compromise investigations by police, preventing successful prosecutions.

"They may have inadvertently put more children at risk through their actions," he added.

He suggested that the right thing to do with online evidence of child abuse is to report it to "appropriate authorities."

He also suggested that people who use the child pornography sites were unlikely to use their own names as usernames, and releasing the names may therefore put innocent people at risk.