British Prime Minister Margaret Thatcher said today that the U.S. vote in favor of a United Nations resolution calling for negotiations over the Falkland Islands was "unbelievable" and said her disappointment is "very deep indeed."

Her voice heavy with emotion, Thatcher told an interviewer for the British Broadcasting Corp.: "We've always been true to the values which we and the United States share--values of freedom, justice and democracy. I find it incomprehensible that the United States should vote with Argentina, a totalitarian government that has invaded a democracy.

"Anger? No. Disappointment? Very deep indeed."

Thatcher dismissed yesterday's non-binding U.N. resolution, saying, "I'm not negotiating with Argentina. We are confident of our title to sovereignty."

Her remarks came in Paris, where she was ending a summit meeting with French President Francois Mitterrand, whose government and 51 others abstained. Ninety nations voted in favor of the resolution; 12 opposed it.

Britain had hoped the United States would also abstain or at least withhold its decision on the vote until the debate was over. Instead, U.S. officials let it be known at the outset that it would back Argentina, a move that stunned the British and raised fears here of what one official called "a bandwagon effect."

Despite Thatcher's bitter remarks about the U.S. stance, British officials said today that the vote at the United Nations had not gone as badly as they feared. The abstentions included most of Britain's partners in the European Community. Nonetheless, they acknowledged that the vote puts Britain somewhat more on the defensive in refusing to discuss the future of the islands with Argentina.

"In our system and in the United States we believe in self-determination," Thatcher said. "That's what put President Reagan where he is in the United States, and that's what put me where I am in the United Kingdom. And in the end it's the fundamental human rights that matter. . . .

"We will not change our fundamental obligation to the British people in the Falkland Islands."

Thatcher's comments recalled a similar public complaint in September over President Reagan's decision to impose sanctions against a British company for defying the U.S. ban on supplying equipment to the Soviet natural gas pipeline. She said then that she felt "deeply wounded by a friend."

The tone and content of her statements today reflected a more personal sense of rejection. Thatcher had appealed to Reagan on Monday not to vote for the resolution, so his decision to authorize the step was a direct rebuke to the British position.

While criticizing the U.S. vote, Foreign Secretary Francis Pym in a separate broadcast interview today stressed that Britain remained grateful to the United States for its support during the conflict, which followed Argentina's seizure of the Falklands April 2. Pym and other officials seemed eager to minimize the strain caused by what one called a "hiccup" in the otherwise "extremely close" Anglo-American relationship.

Thatcher, however, has made the Falklands cause a political crusade, and there was no mistaking her resentment against the U.S. vote. "I find it unbelievable," she said. "I can't account for it at all."

Some officials suggested today, half in jest, that Britain might retaliate, perhaps by refusing to go along with the U.S. rejection of the law of the sea treaty governing the international control of seabeds and minerals. But one knowledgeable official said such a move was highly unlikely.

"The law of the sea question is too important for us," he said. "We'll make up our minds on whether signing is in our interests, not on a matter of pique with the U.S."