Basque extremists, concious of their growing isolation following a wide ranging home-rule agreement for the area, apparently have adopted indiscriminate terror tactics in an effort to provoke a reaction in conservative, particularly military, circles.

Sunday's bombings in Madrid's two main railroad stations and the capital's internation airport signaled a departure from the tactics previously used in the 10-year-old campaign of ETA, an acronym for Basque Homeland and Liberty, to set up and independent Marxist-Leninist Basque republic.

The three synchronized blasts, which killed five persons and injured at last 113, came at a time when million of Spaniards leave the cities for their annual summer holiday. It was the first time ETA deliberately had struct at civilians, and the resulting outrage among Spaniards is widespread.

The political-military wing of the Basque separatist guerrilla organization ETA claimed responsibility for the bombings. A comminique issued in Paris sunday night blamed Spanish authorities for failing to clear the areas in time after warnings were called into news media, and it said the group would plant more bombs in tourist centers.

Meanwhile, bomb threats closed Madrid's subway system for more than an hour today and forced police to clear the capital's central Chamartin railroad station -- the scene of one of Sunday's blasts -- and a bus depot. Hoaxes also were reported at the airport and other transportation centers.

Touring the bloodstained wreckage at Barajas International Airport, Madrid's mayor, Enrique Tierno Galvan, said the bombs had not come as a surprise since a violent reaction by ETA had been expected after the home rule agreement between moderate Basque politicians and the government.

In the past, however, ETA has struck at policemen, military officers, Basque rightists and alleged informers. Until Sunday it had not been indiscriminate in its bombings.

When ETA blasted Spain's Mediterranean beaches in June in a bid to sabotage the tourist industry, small explosives were used, ample warning was given and there were no victims. The bombs Sunday in Madrid were 12-pound charges let in luggage lockers and the 40-minute warning relayed to a news agency gave police little time to act.

A government adviser said, "it's impossible to rationalize bombs against people leaving on holiday. This is ETA gone mad. It's ETA having a final desperate fling."

The Basque home-rule statute negotiated by Premier Adolfo Saurez's centrist government and the moderate Basque Nationalist Party, the largest Basque party, July 17 removed the main planks of ETA's political platform in its struggle for secession.

The agreement grants a future Basque government, which is due to be set up this winter after the approval of the statute in a local Basque referendum in October, wide ranging powers over education, finance, industry, transportation and security in the area.

Behind the statute are not only the Basque moderate Nationalists and the government party, but also the socialist and Communist parties and significant smaller radical Basque parties -- in other words, the whole Basque political front organization known as Herri Batasuna (People's Unity).

The government's explanation for the ETA bombing campaign is that since the vast majority of Basques appear to support the home-rule statute, ETA is increasingly isolated and feels it has to break up the agreement whatever the cost.

Thus ETA has been sniping constantly at the administration and the police and military hierarchy, claiming about 40 victims out of a total of 91 dead in political violence in Spain so far this year.

The murders of four paramilitary policemen on patrol in the Basque country in the 24 hours preceding the Madrid bombs fell into the familiar pattern of ETA hit-and-run attacks. The policemen were shot from passing cars in three separate incidents.

But the bombs the following day demonstrated that ETA feels these tactics will not suffice. In communiques since the bombings, ETA has threatened to continue the terror, stating that tourist resorts could once more become targets.

At the end of the road, ETA strategists apparently envisage a rightist backlash, perhaps the intervention of the military and a suspension of constitutional guarantees and certainly second thoughts in Madrid governing circles about the Viability of home rule for the Basque country.

So far the government has maintained an outward calm. In a measured official statement after the bombs it said that the outrage was "a desperate provocation against the proposals for peace in the Basque country, which are enshrined in the Basque home rule bill." The Statement reaffirmed the government's intention to see the bill through.

With the war of nerves and terror now officially declared in the campaign leading referendum on Basque home rule, the government will need all the backing it can get to offset emotional reactions among Spaniards and the histronics of the rightists.