Bomb disposal experts have been mobilised as officers and sniffer dogs search several properties in Sant Pere de Ribes near Barcelona.
Regional Mossos d’Esquadra officers are expected to announce arrests later this morning from a series of dawn raids targeting Islamic terrorism.
The force tweeted early today: “An operation is underway in Sant Pere de Ribes.
“Several units are carrying out raids and arrests are expected throughout the morning.”
It added in a second tweet: “The operation is the culmination of an investigation which has been going on for months.
“It follows the detection of web users who in a coordinated fashion, praised and justified the actions of terrorist groups as well as inciting others to commit terrorist acts.”
Sant Pere de Ribes is around an hour south from Barcelona, near Sitges.