London, Nov 7 (IANS): A raid involving 16 European countries and the US led to the shutting down of more than 400 websites selling illegal items including drugs and weapons, a media report said Friday.

The joint effort saw 17 people arrested for their involvement in the operations of the illegal sites, which could be accessed only on the Tor network -- a part of the internet unreachable via traditional search engines, BBC reported.

Tor is believed to be home to thousands of illegal marketplaces, trading in drugs, child abuse images as well as sites for extremist groups, the report said.

"Today we have demonstrated that, together, we are able to efficiently remove vital criminal infrastructures that are supporting serious organised crime," Troels Oerting, head of Europol's European cyber crime centre, was quoted as saying.

"And we are not 'just' removing these services from the open internet; this time we have also hit services on the dark net using Tor where, for a long time, criminals have considered themselves beyond reach," he added.

The so-called deep web -- the anonymous part of the internet -- is estimated to be anything up to 500 times the size of the surface web, the daily said, adding that there were approximately three million Tor users but the number of sites may be smaller.