Angry rightist crowds in the Basque city of Guernica today turned the funeral of a high provincial official slain by Basque separatists into a demonstration against Premier Adolfo Suarez and his policy of amnesty for Basque political activists and sel-rule for the restive region.

The Roman Catholic service was interrupted by shouts of "Suarez resign, you have sunk Spain," and "Power to the army." In recent weeks, senior military officers who fought for the late dictator Francisco Franco in the 1936-39 civil war have warned the premier against concessions to the Basque provinces, claiming that they seek to secede from Spain.

Yesterday's machine-gun killing in Guernica of Augusto Unceta-Barrenechea, 53, president of the Vizcaya Provincial Council, and two of his bodyguards, has raised tensions and fears not only in the northern Basque region but also in Madrid.

Responsibility for the murder was claimed today by ETA, the Marxist-Leninist Basque underground that has rejected the government's amnesty bill and offer of negotiations for self-rule. An ETA communique said it will "continue to attack all the tools used by the oligarchy for dominating the Basque county."

The underground group said that "democracy does not exist in the Spanish state," contending that the country has the same military dictatorship" it had while Franco lived "but now it wears a smile."

ETA's position has been strongly rejected by Basque moderates, particularly by the Basque Nationalist Party which has accepted the government's amnesty bill and is participating in negotiations for self-rule. The underground, however, has support among small Basque parties seeking independence for the Basque provinces of Alava, Guipuzcua, Navarre and Vizcaya.

The Basque Nationalist Party had a delegate at the emotional funeral. ETA and other parties considered the slain official one of their strongest enemies, and had sent him threatening letters.

While he was mayor of Guernica 10 years ago, Unceta-Barrenechea gave Franco the city's medal and made him an honorary citizen.

Unceta-Barrenechea also opposed a government ruling permitting the display of the Basque flag throughout the region.

Crowds at the funeral today waved the Spanish flag. Afterward they roamed Guernica singing a fascist hymn, and raising their arms in the fascist salute. A few fist fights between rightists and nationalists were reported.

Soviet Ambassador Yuli Vorontsov said yesterday that he wants the United States to avoid the "mistake" of pressing too hard on human rights.

Vorontsov, who served 12 years in the United States as a diplomat, told reporters he has been deliberately low key during the first week of the Belgrade review conference on European security and cooperation."

"I want to help you avoid making a mistake," he said. "The European countries have already realized this so-called human-rights campaign is leading nowhere, and the United States is starting to realize it as well."

The conference, which is reviewing implementation of the final act of the European security conference signed in Helsinki two years ago, resumes Monday.

The conference will meet in closed plenary session following last week's open sessions in which initial statements of position were made. The talks are scheduled to last three months.

Sources said th United States will continue its effort to introduce discussion of the human-rights issue during the closed sessions.

U.S. delegate Arthur Goldberg has said he wants to avoid "confrontation" with the Soviets, while at the same time seeking a discussion of charges that the Soviets and their allies have violated the human-rights provisions of the Helsinki accords.