The social media platform used by a Moroccan promoter of terrorism has a terrifying 12,000 users.
Mouner El Aoual was arrested in Turin in the north-western Piedmont region of Italy.
He had been actively recruiting people to help him carry out terrorist attacks, said officers from Italy’s Raggruppamento Operativo Speciale (ROS, special operations group) and the USA’s Federal Bureau of Investigation (FBI).
Using the Zello social media site’s "State of the Islamic Caliphate" chat room, he had a horrifying 12,000 potential terrorists to choose from.
A judge issuing his arrest warrant called El Aoual "extremely dangerous".
He added the Moroccan was at "high risk of moving to the execution of serious acts of violence".
The exact nature of his terror plot, now foiled, was not released.
El Aoual promoted the ideology of the Islamic Caliphate on the web and encouraged attacks on "unbelievers ", according to reports.
He is suspected of crimes aimed at international terrorism and incitement to crime and aggravated crime.
El Aoual, using the name "Salah Deen" on social media, had shared jihadist propaganda images that were difficult to find, an element that indicated his potential involvement in the IS terrorist organisation, authorities said.
They said that on an internet chat El Aoual had expressed a wish to plan a terrorist attack in Italy and was looking for other "soldiers" to help realise his plans.