The Spanish Interior Ministry says two suspected members of Basque militants Eta, arrested in Valencia on Thursday, were planning "imminent attacks".
Police discovered a pistol and some explosives during the raid
Inside the Valencia hostel where the man and woman were staying, officers found a pistol, explosives and a list of potential targets across Spain.
Police cordoned off the hostel, near City Hall, and evacuated the area.
It follows last week's Madrid car bomb and the arrest of an Eta suspect accused of plotting to shoot the king.
The Spanish Interior Ministry named the two suspects as Mikel Orbegozo Etxarri, 25, and Sara Majarenas Ibarreta, 24.
A spokesman said they two were believed to be on the Eta payroll and supplied weapons and explosives to the group.
When the police approached Mr Etxarri on Thursday, he pulled a pistol from a holster and aimed to fire at the officers, a statement said. But he was arrested without shots being fired.
Ms Ibarreta was then arrested in the hostel in the Pasaje de la Sangre, which is often used by foreign visitors to the eastern city.
The car bomb which exploded in Madrid last week injured about 40 people and is believed to have been the work of Eta.
On Wednesday, investigating Judge Baltazar Garzon ordered the detention of Eta suspect Javier Perez Aldunate for allegedly plotting to assassinate King Juan Carlos with a rifle last year. Another suspect was also detained pending further investigation.
The two men were arrested last week along with 15 others as part of a government campaign to destroy Eta's recruitment network.
Police are reported to have found letters from an Eta leader in France saying the group needed to start "killing people as soon as possible" to raise morale among its members.
The arrests followed the Madrid car bomb explosion on 9 February which exploded hours before King Juan Carlos and the president of Mexico, Vicente Fox, were due to visit a nearby building.
Before the arrests in Valencia on Thursday, Prime Minister Jose Luis Rodriguez Zapatero told Spanish radio there had been no contact with Eta and any negotiation required an end to terrorist violence.
He warned Eta and its supporters that there was no place for them in Spain's democracy.
Interior Minister Jose Antonio Alonso, on a visit to Morocco, told reporters Spain needed to "keep its guard up, keep applying political pressure and continue the strategy of absolute determination in the fight against the terrorist organisation Eta".
Eta has been blamed for more than 800 deaths since the 1960s in its battle to form an independent Basque homeland.