WASHINGTON - Seventy-two people have been charged with participating in an international child pornography network that prosecutors say used an online bulletin board called Dreamboard to trade tens of thousands of images and videos of sexually abused children.

Attorney General Eric Holder and Homeland Security Secretary Janet Napolitano said Wednesday a 20-month law enforcement effort called Operation Delego targeted more than 600 Dreamboard members around the world for allegedly participating in the private, members-only Internet club created to promote pedophilia.

Numerous participants in the network sexually abused children ages 12 and under, produced images and video of the abuse and then shared it with other club members, according to court papers released in the case.

At a news conference at the Justice Department, the attorney general called the criminal activity a "nightmare" for the children and said that some of the children featured in the images and videos were just infants.

In many cases, the children being victimized were in obvious, and intentional, pain - even in distress and crying, just as the rules for one area of the bulletin board mandated, the attorney general said.

Fifteen arrested Dreamboard participants personally created child pornography, according to the Justice Department.

Napolitano said the amount of child porn swapped by participants in the network was massive, the equivalent to 16,000 DVDs. Assistant Attorney General Lanny Breuer, who heads Justice's criminal division, called the criminal enterprise "a living horror."

Of the 72 charged in the United States, 43 have been arrested in this country and nine abroad. Another 20 are known to authorities only by their Internet names and remain at large.

Authorities have arrested people in 13 other countries - Canada, Denmark, Ecuador, France, Germany, Hungary, Kenya, the Netherlands, the Philippines, Qatar, Serbia, Sweden and Switzerland, but some of those were arrested on local rather than the US charges.

To conceal their conduct, members used screen names rather than actual names and accessed the bulletin board via proxy servers, with Internet traffic routed through other computers to disguise a user's location, according to the court papers.

Participants were required to continually upload images of child sexual abuse to maintain their membership.

Participants who molested children and created new images of child pornography were placed in a "Super VIP" category that gave them access to the entire quantity of child porn on the bulletin board, the court papers stated.

A "Super Hardcore" section of the bulletin board was limited to posts showing adults having violent sexual intercourse with "very young kids" subjected to physical and sexual abuse.

All 72 US defendants are charged with conspiring to advertise and distribute child pornography, and 50 of them are also charged with engaging in a child pornography enterprise. Thirteen of the 52 defendants who have been arrested have pleaded guilty in the conspiracy. Of the four who have been sentenced, the least amount of prison time was 20 years behind bars and the most was 30 years.