Opinion polls up until the last six days of the campaign showed the ruling Popular Party (PP) winning the general election, but nobody knows how the worst guerrilla attacks in Spain's history would affect the outcome.

Two hundred people were killed and nearly 1,500 wounded by 10 simultaneous bombs that exploded on four packed commuter trains in the Thursday morning rush-hour, drawing more than 11 million people onto the streets in protest on Friday.

Voting started with more than 250 people still in hospital, at least a dozen of them fighting for their lives. Officials ruled out delaying the vote.

Most voters are choosing between the PP's Mariano Rajoy -- the hand-picked candidate of departing Prime Minister Jose Maria Aznar -- and Socialist candidate Jose Luis Rodriguez Zapatero, who opposed Aznar on supporting the Iraq war but has backed him in the fight against armed Basque separatists ETA.

Until the tape was found, Spanish authorities had insisted ETA was the prime suspect for the bombings, drawing thousands to protest at Popular Party offices across Spain.

"Your war, our dead!" protesters chanted at the Madrid headquarters, many convinced the bombs were revenge for Spain's support of the war in Iraq even before the purported al-Qaeda claim.

In Madrid, demonstrators took their protest to several monuments throughout the night. In their neighborhoods, people banged on pots and pans in a traditional form of protest.

An editorial cartoon in left-leaning El Pais showed a voter at the polls wondering whether to choose between ETA and al-Qaeda.

"We don't want it to be al-Qaeda, just as the Basques don't want it to be ETA. But what difference does it make, knowing who it was? We're all affected," said Ahmed, a 38-year-old Muslim living in Madrid.

At an after-midnight news conference Interior Minister Angel Acebes said a purported spokesman for al-Qaeda had claimed responsibility in a videotaped statement for the bombings.