Bomb disposal experts have also been mobilised as officers search several properties in Sant Pere de Ribes near Barcelona.
The operation is targeting Islamic terrorism.
Regional Mossos d’Esquadra officers are expected to announce arrests later this morning from a series of dawn raids.
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.”
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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, just inland from the holiday resort of Sitges, is an hour’s drive south of Barcelona.
Moroccan-born Younes Abouqaaqoub killed 14 people including British-born Julian Cadman, seven, and injured more than 100 others after driving his van along Barcelona’s famous Las Ramblas boulevard on August 17.
He stabbed another man to death during his getaway.
Hours after the Ramblas attack, five other members of the gang ploughed into pedestrians in Cambrils before going on the rampage with an axe and large knives and stabbing one person to death.
The five were shot dead by police.