With U.S.-Soviet nuclear arms negotiations at a turning point, President Reagan's proposals for an advanced technology anti-ballistic missile defense have been received in Western Europe as a setback to hopes that the superpowers can stabilize the nuclear balance.

Because it took Europeans by surprise, official response to the president's plan has emerged slowly. Defense Secretary Caspar W. Weinberger, who met on the eve of Reagan's speech last week with his NATO counterparts, did not brief them on its contents, diplomatic sources said today. As a result, the sources said, concern over the failure to consult on so important an initiative has influenced consideration of its substance.

Public comments by the governments of West Germany and Britain, two of the countries scheduled to receive new American medium-range missiles this year, were cautious, tending to stress the long-range nature of the plan. Privately, these governments and others expressed serious concerns over the timing of the move, its effect on the arms race and the implications for Europe's own defenses.

Whatever the conceivable merits of developing a space-age strategic defensive system, European officials say that there is already so much on the nuclear agenda that these proposals "create even more confusion in an already confused situation," as one German diplomat put it.

With arms talks deadlocked, both the allies and the Soviets were awaiting details of a revised Reagan position on deployment of medium-range nuclear weapons to be announced this week when the president abruptly swerved into a wholly new area. As viewed from here, Soviet leader Yuri Andropov's bitter denunciation over the weekend of Reagan's remarks reflected Moscow's suspicion that the president deliberately upstaged the Geneva arms talks by introducing a new strategic factor outside the bargaining.

Reflecting an opinion also voiced by officials who asked not to be identified, Robert O'Neill, director of the London-based International Institute for Strategic Studies, said a stable nuclear balance would be unobtainable while a U.S. antimissile plan that faces "enormous technological problems" is carried out. "Where does that leave the Soviets?" O'Neill asked, suggesting that the answer is a Kremlin even more disposed than now to countering each American move with one of its own.

O'Neill said he had "detected no favorable comment" in Europe on the president's initiative and said it "heightened a general feeling of apprehension, making the climate worse" for the already controversial upcoming deployment of cruise and Pershing II missiles.

A senior West German official said that the widespread European impression that the president "says dangerous things" is likely to be reinforced by Reagan's renewed insistence in recent weeks that a growing Soviet military threat has to be neutralized by the United States. By this reasoning, the antimissile proposals are seen as further evidence of superpower tensions rather than a long-term effort to reduce them.

Summarizing these concerns, the French newspaper Le Monde said in an editorial Friday that "in the short run," Reagan's plan is "destabilizing." Over the longer term, the editorial asked, "won't the side with this power be tempted to use its nuclear arms, convinced of its impunity. And wasn't it to avoid such a situation that the two superpowers" signed the 1972 accord limiting such defensive systems?

Perhaps most disturbing of all to the Europeans are the implications for future U.S. defense policies. As understood here, an advanced antiballistic missile system would apply only to strategic nuclear weapons and not the low-flying cruise or battlefield nuclear artillery.

American reliance on the ability to destroy Soviet long-range missiles in flight would therefore reopen the question of how far the United States would go on behalf of its allies who would still be under the threat of medium-range Soviet weapons. "If you have this protection for yourselves," a senior British diplomat observed, "defending Europe becomes harder to justify."

Moreover, this diplomat and others said, the independent nuclear deterrents of France and Britain would presumably be even more vulnerable to Soviet attacks than they are now because American strategic targets would be beyond the Soviet reach.

In the final analysis, said one specialist on East-West issues in Bonn, Reagan's speech last week was directed primarily at a domestic audience to increase pressure on Congress to approve his defense budget. "There is a feeling in Europe," he said, "that this proposal was made public without regard to European sensitivities."