MADRID, The Spanish government's historic decision granting a measure of self-rule to the Basque provinces last week appears to have given the moderate an advantage over more violent, separatist groups.

But there has been no celebration and no relaxation of tension in Euskadi - the Basque language name for Spain's most violence-plagued area. Trouble continues to simmer, the latest being a demonstration yesterday in San Sebastian demanding freedom for a couple arrested in connection with theft of explosives.

The Basque Nationalists and the Socialists - the Basque country's major parties - have threatened to reject the much-hailed "pre-autonomy" agreement in a dispute with the Madrid government's Center Democratic Union Party over the future of Navarre. At issue is a referendum in which the people of the province will decide whether they want to be part of the Basque autonomous region.

The relatively moderate Basque Nationalists and Socialists, who have repeatedly comdemned violence as a means of achieving autonomy, were the architects of the complex self-rule agreement announced by Spanish Premier Adolfo Suarez on New Year's Eve after three months of antricate stop-and-go negotiations.

Basque Nationalists and Socialists parliamentarians met yesterday in San Sebastian to parcel out the 15 seats in the General Council which will run the Basque provinces until a final autonomy law is approved by Parliament. Nationalists and Socialists are vying for the council's presidency.

The self-rule agreement with Basque legislators elected in last summer's parliamentary voting raised the posssibility that other Spanish regions will soon obtain self-rule. The draft giving the northwestern Galicia province provisional self-government is almost finished. Canary Islands parliamentarians are meeting this week to consider how to achieve regional rule. The Canary Islands are 823 miles Southwest of Spain in the Atlantic Ocean.

Catalonia not only obtained a regional government last fall, but is well on the way to working out a transfer of powers with central government.

The movement toward regionalism has shocked supporters of the late dictator Francisco Franco, who believe that Spain is being dismembered. The future of Spain's regions, however, will be decided in Parliament and by the municipal elections, which may be delayed until next fall.

Creation of the provisional Basque government did not bring jubilation to the region, but even separatists who elected a senator and a deputy in June's election asked for at least one seat in the General Council. The government's Center Democratic Union Party will have at least three representatives on the council.

There is no question that achievement of self-rule brought a wave of relief throughout the Basque provinces and applause throughout Spain. It serve to eliminate the fear that new violence in the provinces could stifle Spain's budding democracy.

The fear and pressure from politicians forced the government to press for an agreement before the end of the year. The basic draft had been ready since November.

The stumbling block was whether Navarre, still run by a provincial government appointed by the late dictator Francisco Franco, should be included in the Basque region along with Alava, Guipuzcua and Vizcaya.

Members of the government party, which made a strong election showing in Navarre, were against making the province part of the agreement. They argued that Navarre is not Basque.

But at the last moment all negotiators agreed that Navarre should decide whether it wants to be part of the Basque region after municipal elections. The decision taken by the elected provincial government will in turn be taken to the people of Navarre who will approve or reject it in a teferendum.

Navarre, which sided with Franco in the civil war, is deeply divided between those who consider themselves Basques and those who do not. The province's establishment is extremely conservative, and so are many southern Navarre residents. But in the past few years the province has become highly industrialized and it has become the scene of violent conflicts.

The Basque country, including Navarre, has a population of less than 3 million, but the region has played a major role in Spanish history. Many Basques don't consider themselves Spanards. This feeling was recently expressed by Telesforo Monzon, former war minister of the exiled Basque Republic.

The statement raised an outcry throughout Spain because it was believed to raise the specter of separatism favored by ETA, the Marxist Leninist Basque guerrilla underground, and by small political groups that advocate violence and independence.

The president of the exiled Basque Republic, Jesus Maria de Leizaola, the Lekendari or political leader of the Basque nationalist party, has approved the self-rule agreement and asked Basques to eschew violence. He will, however, return to Spain in the immediate future and will have no participation in the Basque regional government.