Rescue workers on Sunday searched for two people missing in the rubble of a powerful car bomb blast that was blamed on the Basque separatist group ETA and shattered a nine-month-old cease-fire, officials said. In Madrid and many other Spanish cities people rallied at noon to condemn Saturday's attack at a glittering new terminal at Madrid's international airport. The blast caught the government and much of the country by surprise and ends 2006 on a somber, ominous note. "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 plaza. He said Prime Minister Jose Luis Rodriguez Zapatero should have the "courage" to 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. The Interior Ministry disclosed Sunday that the van used in the bombing was stolen at gunpoint in France on Wednesday by three people who identified themselves as ETA members and held the vehicle's Spanish owner captive for three days. They freed him in France an hour after the bomb went off in Madrid, the ministry said in a statement. At the airport, crews were using heavy machinery to remove 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 separate 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 500 and 800 kilograms (1,100 and 1,800 pounds) of explosives, said Alfredo Prada, the vice president of that administration. That is a huge amount even by ETA's standards. In the past its car bomb attacks have sometimes involved less than 10 kilograms (20 pounds) of explosive. The blast was so powerful that officials hold out little hope the two missing men are 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. They have complained that the government has made no gesture to reciprocate for the cease-fire, such as meeting a long-standing ETA demand for its prisoners to be moved to the Basque region from other parts of the country. The group also has said that continued arrests of suspected members and court rulings against the movement have broken what ETA calls a government promise to relieve pressure on the pro-independence group. It is also angry that the government has refused to allow talks among Basque political parties on the region's future until ETA's outlawed political wing, Batasuna, renounces violence. The head of Batasuna, Arnaldo Otegi, said Saturday after the attack that he did not consider the peace process dead. "Not only is it not broken, but it more necessary than ever," he said. But Otegi did not condemn the blast. "What happened in Madrid, if it's confirmed ETA is behind it, doesn't take us back to the scenario that existed before March 24," he added, referring to the day ETA's cease-fire took effect.