Spain has been placed on maximum terror alert ahead of official campaigning for the general election scheduled for 9 March, the interior ministry said.
Zapatero has adopted a hard line against Eta since a ceasefire ended
Security forces have been mobilised to protect rallies, party headquarters, shopping centres and other sites.
Spain had been on "medium" alert over fears of an attack by the armed Basque separatist group, Eta, which ended a 15-month ceasefire in June 2006.
Correspondents say the threat from Islamist militants also continues.
The outcome of the last general election in March 2004 is perceived by many to have been altered by the bombing of packed commuter trains in Madrid three days earlier.
On 11 March 2004, the governing Popular Party (PP) appeared to be poised for victory. But within hours, 191 lives had been claimed by an Islamist terror attack.
Three days later the PP was voted out of office, after wrongly blaming the Madrid train bombings on Eta.
The BBC's Steve Kingstone in Madrid says terror will always be an election issue in Spain, and a fatal attack would once again transform the campaign.
The party leaders are the same this time round as in 2004, but Prime Minister Jose Luis Rodriguez Zapatero will begin his campaign on Friday against a backdrop of a slowing economy.
Eta has resumed its armed struggle for independence
Opinion polls suggest he will have a difficult fight to secure a second term in office against his main rival PP leader Mariano Rajoy.
Mr Zapatero's governing Socialist Party is also facing criticism over its failed attempt to negotiate peace with Eta, which has claimed more than 800 lives during a 40-year campaign for an independent Basque state that would span parts of north-western Spain and south-western France.
Since the end of Eta's ceasefire, Mr Zapatero has adopted a hard line against the group.
Previously, he had been criticised by the opposition for seeking talks with Eta.