Governments in Western Europe reacted with skepticism today toward Soviet leader Mikhail Gorbachev's declared freeze on SS20 missiles.

But the timing of the initiative, coinciding with traditional Easter weekend peace demonstrations, appeared likely to inject new enthusiasm in Europe's flagging antinuclear movement.

The Soviet announcement of a moratorium came at the climax of four days of peace marches in Britain and West Germany, where hundreds of thousands of antinuclear demonstrators staged rallies to protest the continuing deployment of U.S. cruise and Pershing II nuclear missiles in Western Europe.

Gorbachev's first major foreign policy initiative seemed calculated to bolster the antinuclear crusades in order to exert renewed pressure on western governments to renounce new medium-range missiles, which are intended to counter the Soviet SS20 force.

Liberal and left-leaning opposition parties welcomed the initiative as an important step toward the curtailment of nuclear arsenals in Europe.

In Bonn and London, however, officials in the ruling conservative governments rejected Gorbachev's accompanying plea for a reciprocal freeze by the West on cruises and Pershings. They said a joint moratorium would ensure an overwhelming Soviet advantage because current warhead levels on medium-range missiles are nearly 10 to 1 in Moscow's favor.

British Prime Minister Margaret Thatcher, visiting Singapore on an Asian tour, said at a press conference that Gorbachev's offer "does not alter the position in any way." She said, "The consequences of such a freeze would not be balance, which is what we seek, but enormous Soviet superiority. That, of course, is totally unacceptable."

But Denis Healey, foreign policy spokesman of Britain's opposition Labor Party, criticized Thatcher's quick dismissal of Moscow's missile freeze and said "it is a good offer and we should take it up."

He noted that many Europeans feel "very disappointed" that "every time the Russians make a sensible proposal there is a knee-jerk negative from Mrs. Thatcher and President Reagan."

British Liberal Party leader David Steel suggested the western allies accept a freeze on deployment until November but only continue it if there was significant progress in the Geneva arms talks.

That notion was scorned at NATO headquarters in Brussels, where a European official said, "Adopting a freeze would mean asking for all sorts of trouble later on from the peace movement if it became necessary to lift the moratorium. I can see the signs now reading, 'Keep the freeze.' "

In West Germany, the ruling Christian Democrats rejected the Soviet proposal, while the opposition Social Democrats called it "an important signal" that should be explored.

Juergen Todenhoefer, the arms control spokesman of Chancellor Helmut Kohl's Christian Democratic Union, said, "Even under its new leadership the Soviet Union has evidently not given up its plan to gain military hegemony in Europe through its SS20 medium-range missiles."

In the Pravda interview released last night, Gorbachev said the Soviet Union was stopping deployment of the SS20s in Europe until November and he urged the West to follow suit by halting the introduction of cruises and Pershings.

Dutch Foreign Ministry officials said the Soviet declaration will not affect the center-right government's position, established last June, which calls for cruise deployment to proceed if the Soviet SS20 force exceeds the 378 missiles deployed at that time.

NATO contends that the Soviet Union already has deployed at least 414 of the triple-warhead SS20s.

In Brussels, a Foreign Ministry spokesman said the government regretted that Gorbachev's declaration was limited to earlier proposals to freeze the existing deployments.

The Soviet moratorium offer appeared to come too late to influence the content of most speeches at the rain-drenched peace rallies this weekend.

But Joan Ruddock, chairman of Britain's Campaign for Nuclear Disarmament, welcomed the Soviet announcement of a missile freeze in Europe.

"The Americans really have no excuse for bringing more cruise missiles into Britain now," she said.

Jo Leinen, a leading West German antinuclear campaigner, called for protests during Reagan's state visit to Bonn next month.

In Britain, the focus of protests shifted from the Greenham Common cruise missile site to Molesworth Air Base, 60 miles north of London, where the country's next contingent of cruise missiles is destined to arrive. More than 2,000 police were mobilized to patrol a barbed-wire fence ringing the base to keep out an estimated 20,000 demonstrators.

In West Germany, more than 300,000 people showed up at rallies and marches in nearly all major cities during the past four days, according to organizers.

The peaceful demonstrations were highlighted today by a human chain, composed of up to 15,000 people, that encircled a Pershing II missile depot near Heilbronn, where three U.S. soldiers died in an accident involving one of the missiles last January.