Basque nationalists and pro-independence Marxist coalitions have scored a decisive victory in the first democratic elections to a Basque regional parliament, trouncing Spain's ruling Union of the Democratic Center.

Sunday's vote put an end to 40 years of direct rule by the central government and clearly indicated that there was widespread popular support for the separatist guerrilla organization ETA, which seeks to set up an independent Marxist state.

The powerful Basque Nationalist Party, politically centrist but ambiguous on the issue of secession, gained 25 seats in the 60-member Basque parliament, and an independent extremist coalition known as Herri Batasuna (People's Unity) emerged as the second political force in the region with 11 seats.

A second secessionist and Marxist grouping, Euskadiko Ezkerra (Basque Left), picked up six seats -- the same number won by the Democratic Center Party, led by Premier Adolfo Suarez.

Forecasts before the election had given Suarez' party between 12 and 14 seats, and as the results became known today, it appeared that the rout suffered by the ruling party together with the gains for the extremists could lead to a serious destabilization of Spain's experiment with democracy.

The vote for Herri Batasuna had been endorsed by the hard-line military wing of ETA, whose hooded members drew large cheers at the coalition's election rallies. The more ideological, but equally radical, political-military wing of the separatist organization had supported Euskadiko Ezkerra in clandestine radio broadcasts.

The two coalitions, which act as political fronts for the rival wings of ETA, now can be expected to place pressure on the cautious and less radical Basque Nationalist Party to push for maximum demands from the Madrid administration.

Their chief demand concerns the delicate issues of the phased withdrawal of the Spanish police from the Basque country and an amnesty for ETA guerrillas in prison or in exile.

The Basque Nationalist Party is committed to attempting cease-fire negotiations with ETA based chiefly on the amnesty issue, but only if the guerrillas cease terrorist operations as a good-will gesture. There have been 28 political killings in the Basque country this year, 18 of them claimed by ETA.

The Madrid government opposes such negotiations because it is against pardoning terrorists, and some independent ETA commandos may not observe the results of peace talks, choosing instead to hold out for maximum demands such as the immediate right to full Basque self-determination.

The overall nationalist demand for a speedy transfer of power to the future Basque government is the source of bitter disputes between the Nationalist Party and the Suarez government.

The serious electoral setback suffered by the government party is the second in less than two weeks and could be repeated on March 20 when the northeastern region of Catalonia holds its elections to a Catalan parliament. At the end of February, the government scored a pyrrhic victory when it narrowly managed to block wide-ranging autonomy for the depressed southern provinces of Andalusia at the cost of a considerable loss of credibility.

In the Catalan elections, opinion polls are showing strong gains by the local Socialists and Communists at the expense of the government candidates.

The setbacks come as a reaction to Suarez' attempt to slow down the rapid process of decentralizing Spain. The government promoted decentralization until early this year when, in a controversial policy switch, it announced that the moves to transfer power had to be rationalized.

The result has been that provincial passions are stronger than ever and that the traditional Spanish clash between the regions and central authority is once more the all-important political issue.