Brussels — Leonid Brezhnev has forced both NATO and the European peace movement to shift gears.
The Soviet leader's sudden announcement last week of a unilateral freeze in deployment of some SS-20 nuclear missiles aimed at West Europe means:
* The Atlantic alliance must now reply to the Brezhnev proposal. At this week's NATO defense ministers' meeting in Colorado Springs, pressure will be on the United States to come up with a response soothing to West European public opinion.
* The powerful West European antinuclear movement will certainly step up its own pressure against NATO's planned deployment of nuclear missiles to counter the Soviet SS-20s already in place.
Even before the March 16 Brezhnev statement, the increasingly well-organized anti-missile campaign had planned a mammoth drive. A central focus will be demonstrations aimed to coincide with President Reagan's forthcoming voyage to West Europe in June.
The Brezhnev statement brought skepticism last week from both supporters and opponents of the NATO missile decision. Both also stated their intense desire for a US-Soviet nuclear arms accord.
All NATO governments and officials were virtually unanimous in heaping scorn on the Brezhnev moratorium announcement. They pointed out (1) that the Soviets had already deployed 300 SS-20s with a total of 900 warheads; and (2) that the freeze only applied to missiles west of the Ural Mountains while those east of the Urals could still reach West Europe.
The general reaction was perhaps best voiced by a top foreign ministry official in Belgium. The ministry has hesitated to announce its decision on whether to accept the US weapons on its soil as planned.
''Since the Soviet Union has already deployed 300 missiles,'' he noted, ''the threat to us is unchanged and the imbalance in medium-range missiles continues.
''Brezhnev's offer to limit the Soviet moratorium to the European part of the Soviet Union is militarily of little significance. They can still hit Antwerp easily with SS-20s from Siberia. We've heard Brezhnev's proposals for a moratorium several times before and each time it becomes less advantageous to us.''
European officials also indicated they would press the US for a more publicly palatable counter to the Brezhnev declaration than the initial rejection. There is concern that this latest Brezhnev card in the propaganda game be trumped quickly lest it gain credibility during the expected antinuclear demonstrations this spring.
From the other end of the political spectrum, Win Bartels of the powerful Dutch Interchurch Peace Council (IKV) in the Hague, said: ''Our position is rather severe on both the Soviets and the Americans.
''In general we support positive unilateral initiatives because negotiations do not amount to much. The leaders aim their offers at public opinion and we are skeptical when there is a new offer. This one is rather late since the Soviet missiles are already in place.''
He said the antinuclear groups were aiming at a ''real zero-option'' by eliminating all nuclear weapons from Europe. In the meantime, most antinuclear activists in Western Europe are preparing a second phase of their massive campaigns of last year aimed at a denuclearized Europe.
The main thrust of the coming drive will be to build up grass-roots political support in both NATO and the Warsaw Pact against nuclear weapons. Many see the superpower negotiations as a trap leading only to mere limits on weapon numbers.
''We want to show we're not just anti-US,'' remarked the Dutch activist.
In addition, the movement will undertake a number of major street demonstrations and conferences to marshal support. The next such gathering is planned for April in Comiso, Sicily, where Italy's NATO missiles would be based. Major events are also planned for June 6 and 7 in London and Bonn on the eve of President Reagan's visits to those cities.
Large gatherings are also planned for Brussels in June and Vienna in August, which planners hope will attract antinuclear activists from Eastern Europe. The movement also hopes to sponsor antinuclear conferences in conjunction with dissident church groups in Dresden, East Germany, in October; in Romania, where other demonstrations have been held; and perhaps Hungary, Czechoslovakia, and even the Soviet Union.