Tens of thousands of Spanish Basques have staged strikes and demonstrations against the terrorist activities of the separatist organization ETA in a mass public repudiation of the organization that was once viewed sympathetically in Spain's northern provinces as a freedom-fighting movement.

The apparently widespread switch in public attitude was in reaction to the murder of a kidnap victim by the ETA Friday night -- the day after King Juan Carlos successfully ended his first visit as monarch to the turbulent Basque provinces. The Basque Nationalist Party, the majority party in the region that in the past had been reluctant to criticize the ETA, strongly condemned the killing, and party leader Juan Jose Pujana called the ETA actions "a murderous fury that must be eradicated."

The victim was Jose Maria Ryan, chief engineer of a nuclear power plant under construction near the main Basque industrial center of Bilbao, who was kidnaped Jan. 29 by the ETA's so-called military wing. Ryan was killed and his body dumped on a side road near Bilbao last Friday after the nuclear plant, a frequent target of ETA protests in the past, refused the kidnapers' demand that the plant be dismantled.

After heart-rending appeals on television and radio by the victim's wife Josefa Marua and their five small children, an estimated 10,000 people demonstrated last Thursday in Bilbao in an effort to convince the terrorists to spare Ryan's life.

The Bilbao newspaper Deia, normally representative of Basque nationalist opinion, said in an editorial, "This latest murder has brought to a limit every possible feeling of pain and indignation."

The consensus among Basque and Madrid politicians was that the tide had been turned against the ETA, an acronym standing for Basque Homeland and Liberty, and that the loss of public sympathy would greatly diminish the ability of ETA terrorists to move "like fish in water" among the people of the northern Spanish provinces.

ETA claimed responsibility for the majority of the recorded 110 political killings last year in the Basque country, and Ryan was the organization's fourth victim this year. A major factor in the unprecedented protest was believed to King Juan Carlos' visit and conciliatory statements credited with winning over Basque moderates previously suspicious of Madrid's central administration.

Madrid political sources said today that a significant element in the mass reaction was the unequivocal condemnation of the Ryan slaying by the Basque Nationalist Party, within which the ETA began as a youth movement in the late 1950s and later splintered off to adopt terrorist tactics modeled on Latin American urban guerrilla movements.

After Ryan's body was found, the protests increased. Yesterday approximately 90 percent of the work force in the Basque provinces heeded a call by the area's main labor unions to strike in an anti-ETA protest. Major factories, schools, public transportation, banks, shops and local newspapers were closed down for the day in Bilbao.

More than 100,000 people demonstrated last night in Bilbao, and similar peaceful protest marches involving tens of thousands were staged in the smaller Basque towns of San Sebastian and Vitoria.

The Basque nationalist leaders, who share the ETA's goal of Basque independence from Spain, were in the forefront of last night's demonstrations. i

Lawyer Juan Maria Bandres, who has often served as a defense attorney for ETA men on trial said, "This is the end of (the military operational branch of) ETA. The prestige of ETA has never before dropped so low."