Catalan nationalists won a major upset victory in elections yesterday for the first regional parliament in Catalonia since the Spanish Civil War 41 years ago.

The moderate Catalan parties displaced the Socialists as the major political force in the northeastern provinces and humilitated Premier Adolfo Suarez's Centrist Party -- its third drubbing by regional votes in the last month.

Catalan voting mirrored by results of elections to a Basque parliament 12 days ago and underlined the spectacular growth of nationalist parties at the expense of Madrid-based political organizations. Regional rejection of centralist parties points to a major alteration in the political map of Spain, long fixed in a strong centralist format by the late dictator Generalissimo Francisco Franco.

The setback suffered by Suarez, who put his personal reputation at stake with a five-day personal campaign in Catalonia, indicates that the government's present policy of slowing the process of decentralization has boomeranged, exacerbating regional passion and the benefiting national parties.

Today's provisional results showed that the moderate nationalist coalition called Convergence and Union, headed by former banker Jordi Pujol, had won 43 seats in the 135-member Catalan parliament. The heavily favored Catalan Socialist Party, affiliated with the national Socialist Party, won only 33. Suarez's Centrist Party received 18 seats behind the Catalan Communists, who won 25.

A surprise result was the strong showing by the small but historic Republican Left Party, with 14 seats. The Socialist party of Andalusia, drawing its support from immigrant voters from that southern region of Spain, won two seats.

On Feb. 28, impoverished Andalusia ignored government advice to oppose extensive regional autonomy and overwhelmingly endoresed home rule in a referndum.

The Basque elections March 9 showed a further extensive loss of support for the prime minister on the regional issue. The Socialists, who are the largest opposition party in the Madrid parliament and therefore the logical alternative to a Centrist government, also lost ground in the Basque voting.

The growth of nationalist parties indicates that no national-based party may gain significant majorities in future general elections. The tentative two-party system that emerged in 1976 after Franco's rule, with Socialists and Centrists competing for the middle political ground appears to be dismantled. Government formations apparently will rest on pacts with nationalist parties such as Pujol's and the Basque Nationalist Party.

A second outcome of the month-long bout of regional voting is that the government will be hard put to slow the transfer of power to the regions. Its call for a slowdown -- in the interests of economic stability and ultimately, consolidation of democracy -- is believed to reflect increasing pressure from the right and the armed forces. They fear a drift into an anarchic confederation.

Suarez now faces simultaneous tough negotations over the extent and timing of decentralization, conducted with Basques and Catalans reinforced by unequivocal mandates. The Basque negotiations are all the more pressing as the violent ETA separatist guerrilla organization uses assassinations to press sweeping solutions.

An ETA commando narrowly missed killing a leading general in a Madrid bomb attack Wednesday that killed the general's driver. While the moderate Basque nationalist seek wide-randing home rule as a political solution to the terrorist problem, the continuing violvence lends weight to the rightists who demand strong central authority.

The Catalan results put an end to the predominance of the Socialist, who were well entrenched with the labor vote in what is Spain's wealthiest region. It also squashed speculation that the Socialists and Communists could form a regional popular-front government, which would be viewed with great hostility by the armed forces. The Socialist blamed a low turnout of 60 percent among the 4.4 million electorate for their losses.

Pujol, 49, is virtually assured of heading the Catalan government, known traditionally as the Generalitant, when the parliament meets next week. He is expected to offer Cabinet posts to the more radically nationalist Republican Left Party and to form a minority but strictly Catalan executive that will seek to negotiate a wide-ranging and rapid transfer of limited powers from Madrid to Catalonia.

Pujol become a household name in Catalonia at the age of 24 when he was severely tortured by police and sentenced to seven year's imprisonment by a court-martial for singing a banned nationalist anthem at a concert attended by four of Franco's ministers.