MADRID (By Elaine Sciolino) March 13, 2004 — Grief and fury blanketed Spain on Friday evening as the country mourned the victims of its terrorist attack and struggled to discover who was responsible.

From Barcelona to the Canary Islands, millions streamed into streets and city squares in a nationwide cry against the violence that left nearly 200 people dead and more than 1,400 wounded in synchronized explosions on four commuter trains in Madrid on Thursday.

They banged drums, carried candles, donned black crosses, wrapped themselves in flags. "No to terrorism!" they chanted in the main Colón square in Madrid, which was enveloped in rain and cold. "Assassins! Assassins!"

One banner said, "We were all on that train."

The conservative daily El Mundo estimated that more than 11 million of Spain's 42.7 million people took part in marches across the country.

The center-right government continued to assert that the armed Basque separatist group ETA and not Al Qaeda, the terrorist organization, was probably responsible for the attack, although the evidence is confusing and nothing has been ruled out.

The political stakes in uncovering the identity of the terrorists are high. Spain's voters will elect a new government on Sunday. Mariano Rajoy, the handpicked successor of Prime Minister José María Aznar and the candidate of the governing Popular Party, is in the lead. He has pledged to continue the policies of Mr. Aznar, including Spain's full participation in the Bush administration's antiterror and Iraq policies.

"So far, none of the intelligence services or security forces we have contacted have provided reliable information to the effect that it could have been an Islamic terrorist organization," Interior Minister Ángel Acebes said at a news conference on Friday, referring to foreign intelligence agencies.

He disclosed that Spanish investigators had discovered a backpack with an unexploded bomb containing explosives similar to those sometimes used by ETA, a Spanish-made detonator and a cellphone apparently intended to trigger the device. He also said British intelligence doubted the authenticity of a letter published in a London-based Arabic newspaper claiming the attacks were carried out on behalf of Al Qaeda. That could not be independently confirmed.

A Basque newspaper and television station said Friday that they had received anonymous telephone calls from ETA denying responsibility.

"An ETA message has arrived saying that it bore no responsibility for the attack," a newscaster for ETB Basque public television said.

Gara, a pro-Basque nationalist newspaper that is regularly used by ETA to make public announcements, said on its Web site that it had received a call from someone who claimed to speak on behalf of ETA who said it was "in no way responsible" for the attacks.

Eighty percent of Spain's trains ran on time on Friday, but the main Atocha commuter railway station, where bombs exploded Thursday, was evacuated briefly after a bomb scare.

Mr. Aznar's decision to join the American war effort in Iraq and send 1,300 troops there was overwhelmingly opposed by his people. If Al Qaeda is shown to be involved, perhaps as punishment for Spain's involvement in the war, it could affect the election's outcome.

With the party machine behind him, Mr. Rajoy, an uninspiring candidate and five-time minister, has run an extraordinarily organized campaign. Government and party officials went on the offensive against their political opponents to build the argument that ETA was responsible for the terrorist attacks.

Mr. Rajoy has enjoyed a comfortable lead of up to 8 percentage points in recent polls. But a poll published on Sunday by the polling agency Sigma Dos and commissioned by El Mundo showed that the Socialists were gaining on the Popular Party, reducing its lead to 4.5 percent.

"Certain opposition parties are trying to use Al Qaeda because of the Iraq war simply to win an election," Gustavo de Arístegui, the foreign affairs spokesman for the Popular Party, said in a telephone interview. "I think it's repulsive."

Mr. de Arístegui, a former career diplomat who has served in the Arab world and who was once the target of a failed ETA attack, also suggested that the stolen van discovered in the town of Alcalá de Henares with seven detonators and a Koranic teaching tape may have been the work of the Popular Party's political enemies.

"I cannot picture a radical Islamist with a beginner's tape," he said. "Normally it would be someone with more sophisticated material. Anybody could have planted a tape there."

In Alcalá de Henares itself, there was also skepticism. Minutes from the train station there, where the police found the van, people were quick to discard, or at least diminish, the possibility that Al Qaeda was behind the train bombings.

"I don't think it's Al Qaeda," said Patricia Muñoz, 24, who stood outside the clothing store where she works, smoking a cigarette, before starting an attack on ETA. "I think it's ETA. People say ETA is dying. They aren't dying. They were just quiet — quiet and busy."

The Socialist Party, which opposed Spain's participation in the American-led war in Iraq, offered a mixed message. José Blanco, the Socialist Party secretary and campaign manager for the Socialist Party candidate, José Luis Rodrguez Zapatero, accused the government of withholding information about the terrorist attacks because of the election.

"They are going to try to avoid making all the information known until after Sunday," he told reporters on Friday.

Mr. Zapatero stayed out of the debate. "Now is not the time to establish political consequences regarding the authorship of the attacks," he said at a news conference today. "There will be time for that."

But he also called on the government "to avoid any element of tension," to inform citizens in a timely manner, and to assume a leadership role "for the sake of national unity, dialogue and confidence."

Mr. Zapatero has said that if elected he will withdraw Spain's troops from Iraq by the summer unless there is a clear United Nations mandate for their presence.

Mr. Rajoy, meanwhile, urged Spaniards to take part in Friday night's demonstrations and vote on Sunday because, he said at a news conference, "that is saying yes to life, yes to liberty, yes to your country and no to terrorism."

He also made clear that they should vote for him. "I am here asking for your vote and your support," he said.

In one of the most macabre snapshots of the bombings, the Spanish radio station Cadena Ser broadcast a 12-second recording of an unidentified woman who had called a colleague's voice mail after the initial explosion on a train at the Atocha station.

The woman, who survived the blast, was in the process of evacuating as she frantically said: "I'm in Atocha. There's a bomb on the train! We had to. . . ." Two more explosions drowned out her screams.

The death toll climbed to 199 on Friday with the death of a 7-month-old girl. Eighty-four bodies remain unidentified. More than 360 of the more than 1,400 injured remain hospitalized.

During Friday evening's demonstrations of solidarity, Mr. Aznar marched in Madrid at the head of the procession behind a white banner and alongside foreign officials including Prime Minister Jean-Pierre Raffarin of France, Prime Minister Silvio Berlusconi of Italy, and Romano Prodi, the president of the European Union's executive arm.

Prince Felipe of Spain and his sisters, Cristina and Elena, Mr. Zapatero, and Mr. Rajoy were also in the forefront of the march, the first time in Spanish history that members of the royal family had taken party in a demonstration.

Under a sea of umbrellas, people thronged Madrid's grandest boulevards, shrugging off the heavy rain.

"What you feel right now is the people's pain, the country's pain," said Jesús Miguel Gómez, 37, a heating supplier who came from a nearby town with friends. "And in the face of pain, you have to be united."

He added: "It doesn't matter who's to blame. They are all a bunch of terrorists. One does it for God, the other because they think they have a purer blood line."

Some demonstrators directed anger not only at the terrorists but also at Mr. Aznar's government for embracing Mr. Bush in his war in Iraq.

"I feel angry, not at Bush but at my own government for saying yes to war," said Josefa Carretero, 41, an administrative assistant. "If it turns out to be Al Qaeda, you don't have to be a genius to realize that it was because we participated in the war in Iraq."

Condolences from leaders around the world continued to pour in. Citizens from 11 countries were among the dead. In exhorting his countrymen to take to the streets, Mr. Aznar said at a news conference on Friday that "all the forces of democracy and society must stand up" to the threat of terrorism.

He pledged that "no line of investigation is going to be ignored" by his government in identifying and punishing those responsible. Referring to ETA, Mr. Aznar said that it was "reasonable to think that this band" was behind the attacks after 30 years of armed activity.

Lizette Alvarez and Dale Fuchs contributed reporting for this article.