MADRID, Spain (CNN) -- Spain put its security forces on maximum alert on Thursday ahead of the general elections on March 9 -- four years after the Madrid train bombings shook the last elections and continue to reverberate through the conscience of society.
The security alert rose from level 2, which had been in effect since the start of the train bombings trial last year, to level 3 -- the maximum alert, the Interior Ministry said.
Maximum alert means police will be backed by the armed forces in patrolling transport hubs, shopping areas, sporting events, landmark buildings, the headquarters of the political parties and their campaign events, and other potential terrorist targets, the ministry said.
The coordinated bombings on four morning commuter trains killed 191 people and wounded more than 1,800 on March 11, 2004, just three days before the last general elections.
A trial that began in February 2007 ended with convictions last October against 21 defendants, including about a dozen Islamic extremists for their roles in the attacks and also some Spaniards convicted of supplying explosives used in the bombings.
"The Interior Ministry has activated the maximum alert to fully guarantee security for citizens during the campaign process that concludes with the elections on March 9," an Interior Ministry statement said.
Spain has arrested more than 300 suspected Islamic extremists since the train bombings although not all remain in jail, the ministry said.
The largest recent operation was last January in Barcelona, when police detained 14 men, mainly Pakistanis, on suspicion of preparing al Qaeda-style suicide attacks against the Barcelona metro and other targets in Europe. A judge held 10 of those suspects in jail for further investigation, but four were released.
In another case, 30 suspected Islamic extremists went on trial last year, charged with plotting to send a truck bomb into the courthouse that tries terrorism cases. The trial ended in January, and while the three-judge panel debates a verdict, they also released 10 of the 30 suspects, although five of the 10 remain in jail on other charges.
In switching to maximum alert, Spanish authorities have not spoken publicly of other specific targets, but they point out that al Qaeda statements regularly mention Spain, among other countries, as a target.
Socialist Prime Minister Jose Luis Rodriguez Zapatero won an upset victory in 2004 after the train bombings and quickly fulfilled his campaign pledge to withdraw Spain's troops from the U.S.-backed coalition in Iraq, which many saw as a reason why Spain had become a target for Islamic terrorism.
Spain continues to maintain troops in peacekeeping forces in Afghanistan and Lebanon, but the Socialist government says those operations are backed by international mandates.
The leader of the conservative opposition Popular Party, Mariano Rajoy, whom polls showed would be prime minister four years ago before losing to Zapatero, is back for a rematch. He has criticized Zapatero for not being tough enough on terrorists.
Spain's buoyant economy, growing faster than other European nations, has shown signs of cooling just as the campaign begins, and has become a major issue in the race. Most opinion polls now show Zapatero with just the thinnest of leads over his challenger.
The two-week campaign begins officially at midnight Friday.
In addition to Islamic terrorism, Spanish authorities are also on alert against attacks by the Basque separatist group ETA, which is blamed for more than 800 deaths in the past four decades, mainly through bombings and shootings.
Police in Spain and France have arrested dozens of suspected ETA operatives, or their supporters, since ETA officially ended its cease-fire last June. ETA's unilateral cease-fire began in March 2006, but after a massive ETA bomb at Madrid's airport killed two men in December 2006 and destroyed a parking garage, the government considered that the fledging peace process had ended. E-mail to a friend 
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