Nine people were arrested in a series of raids on homes and offices across Europe as authorities banded together to tackle German-language cybercrime platforms, police said on Monday.
Over half of the arrests were made in Germany, where police searched 69 properties, the Federal Criminal Police Office and the central bureau for internet crime said in a joint statement.
The illegal sites on the web's so-called darknet offer a platform for trading drugs, weapons, counterfeit money and forged documents, as well as a meeting place for data fraudsters.
The multinational investigation's key suspect is a 27-year-old man from Bosnia-Herzegovina, who was arrested in that country on Wednesday, the statement said. He is thought to have been the underground forums' chief administrator since 2012.
It added that two German nationals had been arrested. They are thought to have been administrators of the cybercrime hubs.
German investigators were assisted by authorities in Bosnia-Herzegovina, Switzerland, France, the Netherlands, Lithuania and Russia.
Several computers and storage devices were gathered as evidence and police confiscated a reported total of 150,000 euros (163,522 dollars) in cash.