Tens of thousands of Basques took part in strikes and in demonstrations today to protest the death of a terrorist suspect who had been under arrest and allegedly escaped.

Barricades, some built out of burning tires, at the entrance to Bilbao prevented many businesses from opening in the Basque country's main industrial center. San Sebastian, the area's second major city, was brought to a standstill as the general strike call was observed strictly.

Clashes between protesters and riot police were reported throughout the Basque region and also in the adjoining province of Navarre, which has a sizable Basque minority. More than a dozen police injuries were reported.

The focus of the protests, the largest recorded this year, was the mysterious death of Mikel Zabalza, who officials say was a collaborator of ETA, the left-wing extremist organization that for nearly two decades has been conducting a terrorist campaign aimed at creating an independent Basque nation.

Zabalza, a 32-year-old San Sebastian bus driver, was detained on Nov. 26. According to the Civil Guard, the paramilitary security force that is in the forefront of the battle against ETA, he escaped that same morning when he was leading his captors to an alleged arms cache in a remote valley close to the border with France.

For 19 days, the mystery of Zabalza's whereabouts deepened. Suspicions grew among nationalist Basque sectors that he had died under interrogation and that the Civil Guard was covering up.

A San Sebastian judge who began investigating Zabalza's disappearance took evidence last week from fellow Basques arrested with Zabalza and subsequently released. They alleged that they had heard him screaming at a Civil Guard barracks outside San Sebastian and that he had been seen on a stretcher.

On Sunday, when tensions were running high, the Civil Guard found Zabalza's body floating in a river close to the spot where he allegedly had escaped.

Madrid's attorney general, Luis Antonio Buron Barba, said yesterday that an autopsy showed that Zabalza had drowned. He said that the forensic experts had not found any signs of ill treatment and that the body had been in the water for 15 to 20 days.

The autopsy's findings apparently upheld the Civil Guard's account, that Zabalza had broken free inside an unused railway tunnel where he had said there was an ETA arms cache. Although handcuffed, Zabalza reportedly escaped through a hole in the tunnel that led down a steep bank to the fast-flowing Bidasoa River.

Among Basque nationalists, the official version was rejected, and the autopsy failed to defuse the tension. Today's protests highlighted the wide credibility gap that Madrid faces in the Basque country.

Danish lawyer Kereen Helverg, a member of Amnesty International who attended the autopsy on behalf of Zabalza's family, told reporters: "No one will ever know whether Zabalza drowned or was drowned."

According to the Civil Guard, Zabalza was arrested at his home at 2.30 a.m. and was taken from the barracks to the tunnel at 6 a.m., accompanied by two lieutenants and a guardsman. Basque politicians have demanded an inquiry to establish why Zabalza was taken, accompanied by so few guards, to locate an arms cache before daybreak. ETA's arms caches are frequently booby-trapped, and the standing order is to approach them with extreme caution. No arms were found.

Spain's deputy prime minister, Alfonso Guerra, said that the Zabalza developments were being "manipulated" by the extremist group Herri Batasuna (Basque for popular unity), which acts as ETA's political front. Its leaders have organized press conferences in which former detainees, among them Zabalza's girlfriend, who was arrested with him, have given lurid accounts of torture.

Guerra, Interior Minister Jose Barrionuevo and other officials say ETA suspects automatically allege torture. Officials insist that policemen who are found to be mistreating detainees are expelled from the security services.

Today's protests were spearheaded by Herri Batasuna, but they were also backed by the moderate Basque Nationalist Party, the main political force in the area, which in the past has criticized ETA's terrorist methods.

The Basque regional government, which is the local executive under Spain's quasi-federal system and is formed by members of the Basque Nationalist Party, has made urgent recommendations to safeguard detainees in Civil Guard barracks.