Police have arrested two men in a probe into the armed Basque separatist group ETA in Madrid.
They found an explosives-laden car that may have been positioned for attack, say authorities.
The two men were arrested on a street in the Puente de Vallecas district of the Spanish capital and police are looking for a third man who got away.
Bomb disposal experts using a robot forced open the trunk of a white Renault Clio and found a 50-litre pressure cooker full of dynamite.
From the way the car was parked - its trunk was pointed at passing traffic - police believe it may have been loaded and ready to be detonated when a target drove past.
National Police chief Juan Cotino visited the Vallecas neighbourhood and praised his force. "We have managed to save many lives," he said.
Police later raided an apartment they said the suspects had used as a safe house, and confiscated a large amount of explosives and forged identity papers, the national news agency Efe said.
Police found the flat because one of the detainees was carrying a piece of paper with the address written on it, the agency said.