An anarchist group in Spain has been sending bombs to prominent Roman Catholics, hiding the explosives inside packages containing vibrators.

At least two such mini-bombs have been sent by a group that calls itself the Anticlerical Pro Sex Toys Group, according to Spain's state-owned EFE news agency. One device exploded at a postal sorting office, slightly injuring the woman who was handling the package.

"Please accept our apologies," the group said in an email apparently sent to an anarchist website at the beginning of last month. "Next time we won't fail."

The group targeted the Roman Catholic archbishop of Pamplona, Francisco Pérez, and the headteacher of a private school belonging to the ultra-conservative Legionnaires of Christ movement in Madrid.

The archbishop told EFE that he vaguely recalled police taking away a suspicious package containing powder. "We didn't give it much importance, but later it was said to be a bomb," he said.

Police reportedly believe the same group is behind a bomb packed inside a pressure cooker that was left near the public prosecutor's office in Madrid, but failed to explode.

Another rudimentary bomb found in Madrid's Almudena Cathedral, combining shrapnel, explosive powder and a camping gas canister, is also thought to have come from the same source.

The anarchists, who use other names such as the Artisans Club for New Uses for Coffee, claimed to have made a bomb out of an espresso coffee machine packed with gunpowder and shrapnel that was planted at a bank branch.