German police said on Friday that investigators have succeeded in shutting down three internet forums on the so-called "darknet" online marketplace with depictions of the sexual abuse of children.

The sites ("BoyVids 6.0", "Forbidden Love" and "Child Porn Market") had hundreds of thousands of registered users, making them among the biggest platforms worldwide for sharing child abuse images, police said.

The "Darknet" is an overlay platform within the Internet that can only be accessed with certain software and knowhow, and where it tends to be harder to trace or identify users. Though it can be used for more legitimiate purposes it is also very frequently used by criminal networks for black market online trading.

German Interior Minister Nancy Faeser called the bust "an important strike against the spread of horrific images of sexualized violence against children.”

The Federal Criminal Police Office (BKA) — Germany's serious crimes police force responsible for cases that cross state borders — is investigating on behalf of the Public Prosecutor's Office in Frankfurt and the Central Office for Combating Internet Crime.

Four suspects arrested

The BKA said four men were arrested in connection with the sites. Those arrested included a 21-year-old man from the eastern German state of Saxony alleged to have been the main administrator of the sites, which were designed to cloak users' identity.

Two other men, a 44-year-old from Lower Saxony and a 45-year-old from Schleswig-Holstein, were arrested in Germany in November and December.

German police said a fourth man had been arrested in Brazil. Investigators are now trying to identify the users of the sites.

dh/msh (dpa, AP, AFP)