Twenty suspects across Europe have been targeted in dawn raids directed against an alleged network recruiting suicide bombers for Iraq and Afghanistan.
Police apprehended 11 people in Italy while others were held or being sought in France, Portugal and Britain.
Investigators suspect the mainly north African detainees were part of a "jihadist" network.
Italian police said they had seized poisons, remote explosive ignition devices and guerrilla warfare manuals.
Tuesday's raids netted most of those named in 20 European arrest warrants issued by a Milan court.
Arrests were made across northern Italy - where investigators believe the alleged plot was based - including at Milan, Bergamo and Reggio Emilia.
In the UK, two men were detained in London and Manchester, while others were being held in France and Portugal, officials said.
Italian Interior Minister Giuliano Amato told a Rome news conference the "very important operation has decapitated a jihadist network that operates across Europe ... with the aim of preparing men to send to the Iraqi and Afghan theatres", AFP news agency reported.
The suspects, mostly from Tunisia, are also accused of falsifying identity documents and immigration offences.
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