Rescue workers on Sunday searched for two men missing in the rubble of a powerful car bomb blast that was blamed on the Basque separatist group ETA and shattered a nine-month cease-fire.

The Interior Ministry said the van used in Saturday's bombing was stolen at gunpoint in France on Wednesday by three people who identified themselves as ETA members. The vehicle's Spanish owner, who reported the carjacking to police, was held captive for three days and freed in France an hour after the bomb went off in Madrid.

In Madrid and other Spanish cities, people rallied to condemn the attack at a glittering new terminal at Madrid's international airport. The blast caught the government and much of the country by surprise.

"Events like yesterday show yet again that all ETA wants to do is kill," Francisco Jose Alcaraz, president of an association of victims of ETA violence, told a demonstration of several thousand people in the Puerta del Sol, a downtown Madrid plaza.

He said Prime Minister Jose Luis Rodriguez Zapatero should abandon the peace process launched with ETA's cease-fire announcement in March, and not just suspend it as Zapatero announced Saturday after the explosion.

At the airport, crews were removing tons of concrete and metal at a five-story parking lot that was largely destroyed in the explosion, said Javier Ayuso, a spokesman for the city council's emergency rescue services.

Two Ecuadorean men believed to have been sleeping inside two parked cars were missing in the rubble, officials said. The explosion also injured 26 people, most of them with damage to their ears from the shock wave.

Ayuso said it could take days to reach the spot where the van blew up.

Explosives experts with the Madrid regional government estimate the car bomb contained between 1,100 and 1,800 pounds of explosives, said Alfredo Prada, the vice president of that administration. The bomb was unusually large compared with past ETA attacks, which have sometimes involved less than 20 pounds of explosive.

The blast was so powerful that officials held out little hope the two missing men were alive, said Luis Villarroel, an inspector with the Madrid fire department. "There is no possibility," he told reporters at the airport, according to the national news agency Efe.

The blast prompted the government to halt plans for negotiations with ETA after a cease-fire that had been seen as the best chance in nearly a decade to end the nearly 40-year-old conflict in Spain's northern Basque region.

ETA did not claim responsibility for the bombing, but a man who placed a warning call before the attack said he was a representative of the group. Following previous attacks, the group has sometimes waited weeks to claim responsibility.

ETA and its political supporters have been warning for months that the peace process was faltering, complaining that the government has made no gesture to reciprocate for the cease-fire.