The Mateo Morral Commando posted the claim on an anarchist Internet forum Thursday, a day after a device exploded in the Basilica of Our Lady of the Pilar in the northeastern city of Zaragoza while people were in the church, which Gen. Francisco Franco visited several times during his dictatorship. No one was injured and little damage was done.
In February, the little-known group claimed responsibility for another small device in Madrid's Almudena Cathedral, which police defused.
The Pilar attracts thousands of worshippers and tourists.
The group it said it gave warnings to two Zaragoza newspapers, but both publications denied this.