On Thursday morning, Defense Ministry security officers in Madrid discovered a package containing an incendiary bomb, which was safely destroyed.
The defense ministry said Spanish security forces had also intercepted a letter bomb at an air base in Torrejón de Ardoz, outside Madrid.
Mr Pérez said the device intercepted at the air base had been deactivated without being destroyed, something that could help investigators trace the origin of the incendiary material inside.
An incendiary bomb was discovered on Wednesday at Zaragoza-based weapons manufacturer Instalaza, which produces the C90 grenade launcher that Spain supplied to Ukraine.
Margarita Robles, Spain’s defense minister, who visited the Ukrainian port city of Odessa on Thursday, said the letter bombs would not deter Spain from supporting Ukraine’s “just cause”.
The Spanish National Court is investigating the spate of letter bombs as possible crimes of terrorism.
The Interior Ministry said it had ordered increased security measures in public buildings, particularly with regard to postal services.
The ministry also said it had ordered an increase in security around diplomatic missions in the country.
The Russian embassy in Spain said: “Any threat or act of terrorism, especially directed against a diplomatic mission, is totally reprehensible.”