Catalan separatists attack plot, Nine people suspected of planning violent attacks.
The suspects are described as Catalan separatists by police sources, who say they plotted to strike on 1 October.
That will be the second anniversary of a Catalan independence referendum declared illegal by Spanish courts.
Materials that can be used to make bombs were found during raids across the Catalonia region, police say.
But explosive experts have yet to determine whether there was any plan to develop explosives from the materials found, Spain’s acting interior minister, Fernando Grande-Marlaska, says.
The detainees belong to a group that has led recent protests, according to police.
Known as the Technical Response Teams, they are seen as the most militant arm of the Committees for the Defence of the Republic (CDRs).
But they are not known to have plotted or carried out violent attacks before.
Police raided 10 addresses in Barcelona, Sabadell and Santa Perpètua de Mogoda at 05:00 local time (03:00 GMT) on Monday.
Computer hardware and documents were seized, police say, in addition to the arrests. They say they have not ruled out making further arrests.