Europe's changed face after Reagan nuclear policy talk.

President Reagan's peace speech has already changed the face of the antinuclear debate in Western Europe. Whether it will now also change the heart of the debate remains to be seen.

Soviet concern about the impact of the speech was apparent in the unusually swift rejection of Reagan's ''zero option'' offer by the official Soviet news agency Tass within hours of the President's address.

Soviet concern was also apparent in the most crucial field of public debate - West Germany - in the eagerness of senior Soviet journalists to prerecord rebuttals of the speech for local radio stations even before Reagan spoke.

Concern by antinuclear protesters was also evident in the quickness of one of the West German movements' leaders - Social Democratic executive committee member Erhard Eppler - to counter NATO's zero-option proposal for intermediate-range nuclear missiles.

''Zero option'' is President Reagan's proposal not to deploy cruise and Pershing II nuclear missiles in Europe if the Soviet Union will agree to withdraw its nuclear missiles directed at European targets.

Again, even before the President gave his address - which was televised live to very attentive European viewers - Eppler criticized the confining of zero option to land-based missiles. This, he contends, is unfair to a land power like the Soviet Union that naturally puts most of its defense effort into land systems.

What all this means - for the first time since Reagan took office - is that the Russians and Western Europe's most passionate opponents of new NATO nuclear weapons are now on the defensive on the peace issue. It's they who now insist that despite all the appearances, Reagan is not really a man of peace.

Now it is Washington's zero option of major weapons cuts that occupies stage center, and not Moscow's various moratoriums or its freeze on existing weapons.

Reagan's embrace of the phrase ''zero option'' is a major factor in this shift of focus, for the phrase - an occasional slogan of antinuclear groups here - packs emotional appeal.

Just how long-lasting this American seizure of the peace initiative will prove to be is altogether a different question. West German observers at least think the long-term impact of the President's speech will depend greatly on Reagan himself.

If his peace speech was a one-shot affair, if Secretary of State Alexander M. Haig Jr. and Secretary of Defense Caspar W. Weinberger revert to public acrimony about scenarios for the use of nuclear weapons in Europe - and if Reagan and his team again evoke the specter of irreconciliable East-West conflict - the potential new Reagan image will swiftly fade.

If, on the other hand, Reagan now proclaims that under his presidency the United States has already improved its military posture and signaled its toughness to the Russians - and can now approach Moscow with the generosity born of strength - then the new Reagan image might take hold in Europe.

The barriers are still formidable, however. One troubled West German journalist commented, ''I'm from the right (in the West German political spectrum). But I hear things from the Reagan administration that are not rationally explainable.''

As proof that many conservatives share his concern he pointed to a recent public-opinion poll that revealed major misgivings about Reagan's stress on East-West confrontation among conservative as well as Social Democratic West German voters. And he noted the same phenomenon not only among the Dutch, but also among Englishmen, and even Frenchmen he has talked with.

He cited, noncommittally, the dispute between Soviet claims that a nuclear balance already exists in Europe and American claims of a strong Soviet superiority. And it was clear that for him Reagan's rigging of the numbers to show a 6-to-1 disparity (International Institute for Strategic Studies figures show a 2-to-1 disparity in weapons, a 4-to-1 disparity in warheads) cast doubt on, rather than proved, Reagan's main contention.

It is the audience of conservative Europeans like this man and casual antinuclear demonstrators that Reagan will have to convince if the new US peace drive is to prove effective.

The antinuclear leaders and Dutch voters have already written Reagan off.

But other Western Europeans have not, such as the 16 million West German viewers who watched Reagan give his speech just after the West German-Albanian championship football match; the schoolgirls who joined the 250,000-strong Bonn antinuclear march last month because it was the ''in'' thing to do; all the young people who haven't yet sorted out their priorities in protesting pollution , nuclear power, nuclear weapons, the housing shortage, the tyranny of technology, and the loss of authority in the modern world.

The final opinions of this group may now be decided largely by how persuaded they become of Reagan's sincerity in the pursuit of peace.