MADRID, — Rescue workers at the Madrid airport searched Sunday for the bodies of two men believed to be lying under rubble left by a car bomb on Saturday that the government said was the work of the Basque separatist group ETA. Basque separatists insisted that despite the bombing, the peace efforts were not dead.

The Spanish government on Saturday suspended its planned dialogue with ETA after the bomb exploded in the airport parking garage.

Prime Minister José Luis Rodríguez Zapatero, a Socialist whose decision to talk to ETA was criticized by the conservative opposition, said Saturday that ETA would have to show signs of renouncing violence if dialogue were to take place.

But Karmelo Landa, a leading member of Batasuna, the political group linked to the separatists, said: “The process hasn’t collapsed. On the contrary, we think it is very necessary now to address the roots of the conflict.” Thousands of anti-ETA protesters took to the streets of Madrid and other Spanish cities on Sunday.

Rescue workers used heavy machinery to search for two men, both Ecuadorean immigrants, believed to have been buried under tons of concrete brought down by the bomb, which wrecked a multistory parking garage at the ultramodern Terminal Four on Saturday morning.

If their deaths are confirmed, the blast will have been the first attack attributed to ETA to claim a life in more than three years.

The bomb, a van packed with several hundred pounds of explosives, was detonated an hour after the first of three telephoned warnings.

The Interior Ministry said the van had been stolen in southern France on Wednesday. Its owner, a Spanish man on a camping trip, had been held hostage for three days before being released in France an hour after the blast.

Nineteen people were slightly hurt by the blast, which caused panic in Terminal Four, where the police did not begin an evacuation until after the departure hall filled with smoke.

In June, Mr. Zapatero declared the start of peace negotiations with ETA, which stands for Basque Homeland and Liberty in the Basque language. The group has killed more than 800 people in its fight for independence of the northern region, which has a distinct culture and language.

But negotiations never got off the ground. Separatists insisted that the government legalize Batasuna and move ETA prisoners to jails closer to their homes, and the government protested after ETA supporters set fire to buses.

Mr. Landa of Batasuna said Mr. Zapatero had been careful not to say the search for a negotiated solution to the four-decade conflict was over.

“We have noted how careful he was with his words,” Mr. Landa said. “He never spoke of a rupture.”

Referring to Mr. Zapatero, a government official who spoke on the condition of anonymity said: “He closed the door, but didn’t slam it shut. He didn’t say, never again.”

But he said some time would have to pass before the government would be ready to speak to ETA again.

On Sunday, Madrid’s central square filled with thousands of protesters carrying Spanish flags and chanting for Mr. Zapatero to resign.

“The government has to realize it can’t negotiate with murderers,” said one demonstrator, Pablo Garijo, a hotel sales executive, who was holding a large Spanish flag.