Two suspected Basque separatists have been arrested near the Spanish town of Pamplona, days after a bomb was planted in a tourist hotel at the height of the annual bull-running festival.
An ETA bomb was left in Pamplona
Explosives, guns, timers and detonators were also found, officials said.
Media reports said the arrest may be linked to the Pamplona bombing, but officials have not confirmed the suggestion.
A total of 150kg (330lb) of explosives were found in the raid, including 30kg which had been prepared for imminent use, said Interior Minister Angel Acebes.
The haul was found in a flat in the town of Berriozar, not far from Pamplona.
As well as the device in a Pamplona hotel, ETA's other recent attacks in Navarre have included the killing of two policemen in a car bomb in Sanguesa on 30 May.
A series of attacks blamed on ETA has been carried out since elections on 25 May in which Batasuna, labelled by the government as ETA's political wing, was banned from taking part.
The device in Pamplona had been planted in a basement toilet in a town centre hotel.
It was defused after a caller claiming to represent ETA gave a telephone warning.
ETA has been blamed for killing more than 800 people in its long campaign for an independent Basque homeland.