The State Department said yesterday that Chile's plebiscite on Wednesday was unfair, failing to "give the voters a full and fair opportunity to express their views."

In a stinging response to the announced 3-1 backing for President Augusto Pinochet, a State Department spokesman declared:

"While we do not desire to prescribe the internal political system of a foreign country, we believe as a matter of principle that any election matter of principle that any election held should offer all parties sufficient guarantees to present their case."

Political activity, "freedom of the press and assembly are sharply restricted" in Chile, he said, and the vote "was unfairly posed as a choice between 'the dignity of Chile' and 'international aggression.'"

Gen. Pinochet called for a straight yes or no vote on the question: "Faced with the international aggression unleased against our government, I support President Pinochet in his defense of the dignity of Chile and I reaffirm the legitimacy of the government."

Opposition efforts to promote a "no" vote were severely repressed but Pinochet - who called the plebiscite in reaction to a U.N. condemnation of Chilean human rights violations - took the result as a mandate. "There will be no more elections and no more voting for 10 years," he told a cheering crowd.

Despite the declaration, lengthening out previous vague promises of return to elections by 1986, the U.S. spokesman said, "We hope that the events of th past week may mark the beginning of a process that allows Chileans to express their democratic preferences in the future, and there will continue to be improvements in the human rights situation there."

While the statement did not address the question of whether the vote-gathering process itself was fair, officials said that, like journalists on the scene, they had no way of knowing if vote totals from the more than 20,000 voting places were accurately reflected in the announced totals.

Air force Gen. Gustavo Leigh, one of the two members of the ruling four-man junta who opposed Pinochet's decision to hold the plebiscite, said after announcement of the results that foreigners will still believe "we fixed the election. It is always going to be thought that we are violating human rights."

In this country, Sen. George McGovern (D-S.D.) of the Senate Foreign Relations Committee called the vote "a travesty, a farce and an insult to the people of Chile."

Isabel Letelier, the window of assassinated former Chilean Foreign Minister Orlando Letelier, referring to the wording of the plebiscite question, said: "The voters were asked to 'reaffirm' something they had never affirmed, the legitimacy of the Pinochet government." She is an associate of the Institute for Policy Studies and the Chile Committee for Human Rights.

Pinochet seized power in 1973 from a constitutional government and Wednesday's vote was the first since then.

In a dispatch from Santiago, United Press International noted that after the polls closed Wednesday, groups of people went to the home of Christian Democratic President Eduardo Frei, who had opposed the plebiscite, and shouted insults and obscenities.

A bitter Christian Democratic Party spokesman said afterward, "No one who loves liberty can assign any respectability to an election held under a climate of psychological warfare."