Many of the estimated 2.3 million marchers in Madrid huddled against a steady rain in a bobbing mass of umbrellas that clogged the capital's squares and the area around the Atocha station, where two of the four trains blew up during Thursday morning's rush hour.
"It is not raining. Madrid is crying," said Jorge Mendez, a 20-year-old telecommunications student.
At least three Americans were among more than 1,400 people wounded in the bombings, the State Department said as it advised U.S. citizens in Spain to be alert and avoid crowds.
In a show of national unity, massive crowds gathered in Barcelona, Seville, Valencia and even in Spain's Canary Islands off Western Africa. State TV said nationwide, more than 11 million marched — one-quarter of Spain's 42 million people.
Prime Minister Jose Maria Aznar, who was joined by other European leaders as he led one march, pledged to hunt down the terrorists whose bombs sparked new fears about Europe's vulnerability to attack.
The debate over who was responsible for the attacks could affect the outcome of national elections set for Sunday.
Aznar and his government ministers blamed the armed group ETA, which has fought for decades for an independent Basque homeland. But there was concern that Islamic militants and perhaps even the al Qaeda terror network had been involved.
As CBS News Correspondent Richard Roth reports, Aznar insisted there was logic in his immediate claim that the ETA was behind the attacks: a sports bag found on one of the bombed trains, with explosives of the type ETA has used. And just last month, authorities intercepted a van load of explosives bound for Madrid, and arrested two ETA members.
"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 Angel Acebes said Friday.
If ETA is found responsible, that could boost support for Mariano Rajoy, Aznar's hand-picked candidate to succeed him as prime minister. Both have supported a crackdown on ETA's campaign for an independent state in northern Spain, ruling out talks and backing a ban on ETA's political wing, Batasuna.
However, if Thursday's bombings are seen by voters as the work of al Qaeda, that could draw their attention to Aznar's vastly unpopular decision to endorse the U.S.-led invasion of Iraq and deploy Spanish troops there.
Roth reports police say as many as 30 terrorists could have had a hand in Thursday's attacks, apparently using cell-phones to set off at least some of the bombs. And they've reportedly found a clue in the wiring: ETA used aluminum, but copper was found here: an al Qaeda signature. Experts say there may also have been a suicide component -- another al Qaeda hallmark.
Rajoy is 3-5 percentage points ahead of Socialist candidate Jose Luis Rodriguez Zapatero in polls. Polls close Sunday at 2 p.m. EST and exit poll results will be available soon thereafter.
A Batasuna leader, Arnaldo Otegi, accused the government of seeking political gain by blaming ETA. "The Spanish government is lying," he said.
The attack's lethal coordination and timing — 10 explosions within 15 minutes — suggested al Qaeda. But the compressed dynamite used in the backpack bombs is an explosive favored by ETA.
ETA denied responsibility, according to Gara, a Basque newspaper that the armed group uses to issue statements. Gara said a caller claiming to represent ETA phoned its newsroom Friday to deny government allegations that the group was to blame.
It was the first time ETA was known to have issued such a denial. The group normally claims its attacks in statements to pro-Basque independence media several weeks later.
Suspicions of al Qaeda involvement gained weight after police found a stolen van with seven detonators and an Arabic-language tape of Quranic verses parked in a suburb near where the stricken trains originated. A London-based Arabic newspaper also received a claim of responsibility in al Qaeda's name that called the attack "part of settling old accounts with Spain, the crusader, and America's ally in its war against Islam."
In a chilling account of the bombings, Spanish radio station Cadena Ser broadcast a 12-second recording of an unidentified woman who had called a colleague's voice mail after an initial blast on a train at the Atocha station.
The woman, who survived, was in the process of fleeing as she frantically says: "I'm in Atocha. There's a bomb on the train! We had to " and then two more blasts are heard amid her screams.
The death toll climbed to 199 on Friday with the death of a 7-month-old girl. Of the more than 1,400 wounded, 367 people remained hospitalized, about 50 in serious condition. Of the dead, 84 bodies remained unidentified. Since the Sept. 11, 2001, attacks in the United States, only the Bali nightclub bombings that killed 202 people in 2002 have been more deadly.
Workers in surgical masks and cutting torches began dismantling sections of the bombed-out trains, taking samples for study.
The New York City police department sent two people from the intelligence division to Madrid — a bomb expert and a lieutenant who was assigned to Interpol.
Friday night's massive rallies in Madrid, Barcelona, Seville, Valencia and other cities and towns were a remarkable show of unity in a nation divided by regional loyalties and languages.
"We all need to be here to repudiate these killings. All of us. It is our duty," said Manuel Velasco, a university professor who was drenched from the rain.
Marchers held banners reading, "No to Terror" and "Today Our Tears Reach Heaven." Another read simply, "Who and Why?"
"Peace in Madrid and in all of Spain is becoming more remote," said the Rev. Manuel Gonzalez. "We are a passionate people but we want peace."
Before the rallies began, offices, shops and cafes across Spain emptied at noon as people stood in silence on the streets to honor the dead. Authorities had requested a minute's silence but many people in Madrid stood in drizzly, chilly weather for about 10 minutes.
The silence ended when the people broke into spontaneous applause in a traditional sign of respect and solidarity.
Aznar stood outside the presidential palace with senior officials. The silence there was broken when someone angrily shouted: "Send the terrorists to the firing squad!"
In Barcelona, subways and buses halted and construction work stopped. In northern Spain's Basque region, hundreds of students and professors at the University of the Basque Country in Leioa also stood in silence.
"This is to show our rejection of violence and our solidarity with the families (of the dead)," said Mikel Luzuriaga, a Basque medical student.
Underscoring jittery nerves, police hastily evacuated the Atocha train station amid a bomb scare that turned out to be a false alarm.
Mostly though, Madrid was engulfed in grief. Black bows of mourning dotted the city, on shop windows, on flags draped from balconies, and on lapels. Relatives converged on a makeshift morgue, searching for missing loved ones.
Commuters fell silent as their trains rumbled past the bombed-out hulks at Atocha station.
At Atocha, mourners sobbed, lit candles and left flowers as the normally bustling hub turned quiet.
Aznar said 14 foreigners were among the dead: three Peruvians, two Hondurans, two Poles, and a person each from France, Chile, Cuba, Ecuador, Colombia, Morocco and Guinea-Bissau.