US and European police on Thursday announced the shutdown of two huge "dark web" marketplaces that allowed the anonymous online trade of drugs, hacking software and guns.

US Attorney General Jeff Sessions said underground websites AlphaBay and Hansa had tens of thousands of sellers of deadly drugs like fentanyl and other illicit goods serving more than 200,000 customers worldwide.

"This case, pursued by dedicated agents and prosecutors, says you are not safe, you cannot hide. We will find you, dismantle your organization and network, and we will prosecute you," Sessions said in a warning to dark web entrepreneurs.