London — Hovering at the elbows of United States and Soviet diplomats at the Geneva arms-talks table is an invisible but tangible presence: Europe's still-growing peace movement.
Its leaders around the continent tell this newspaper in interviews their movement is by no means deterred (or impressed) by President Reagan's ''zero option'' speech or, indeed, by Soviet leader Leonid Brezhnev's phraseology during his visit to Bonn. They expect little from Geneva. Leaders plan more demonstrations later this month and next year.
The likely impact of all this on the talks, according to well-placed sources here:
1. The Soviets will stall for some time, hoping European NATO allies will force the US to make concessions on missiles that Moscow won't need to match.
2. Strains between Washington and Western Europe will grow. European leaders will be more anxious than the US to discover concessions in Moscow's rhetoric that would allow NATO to reduce cruise and Pershing missile deployment and thus ease political pressures at home.
''Brezhnev hasn't really budged from his moratorium plan,'' says one analyst. ''The Soviet strategy will be to sit tight, to see if NATO shoots itself in the foot. . . .''
Already Chancellor Schmidt of West Germany has warned Brezhnev not to think that the peace movement will do Moscow's work for it.
The chancellor thus conceded that the Soviets must draw comfort from the sight of more than 1 million churchgoers, leftists, trade unionists, environmental ''Green Party'' members and young people streaming through the streets of Bonn, London, Brussels, Amsterdam, The Hague, and other cities in the last two months alone.
Leaders of the Dutch Reformed Church, of the Roman Catholic group Pax Christi , and of other peace movements plan a conference of 1,000 officials in Brussels just before the annual NATO meeting there this month.
''Our membership,'' said Bruce Kent, a Roman Catholic monsignor who is general secretary of Britain's Campaign for Nuclear Disarmament (CND), ''is rising now by 300 to 350 every week.''
Leaders in Amsterdam and Antwerp also said the peace movement was still gaining momentum. Indeed, it helped produce the Geneva talks themselves.
Leaders predict confidently that the 96 cruise missiles planned for Belgium and the Netherlands by 1984 cannot now be deployed. Many a pro-missile analyst privately agrees.
Peace group officials also report 100 protest committees formed in Sicily, where 112 cruise missiles are to be deployed by 1984. Italy is important: Schmidt says West Germany will take new missiles as long as one other continental NATO ally does so. If Belgium and the Netherlands reject them, only Italy is left. (Britain plans to take them but is off the continent.)
Feeding the peace movement are other concerns: young people frustrated at massive unemployment in Europe and using protests as a barometer of their overall concerns. Talk of a warning nuclear shot by Secretary of State Alexander M. Haig Jr. (later contradicted by Defense Secretary Caspar Weinberger). Richard Allen's leave from his position as national security adviser at the White House amid charges of impropriety adds to European uneasiness.
''A large part of the peace movement sees zero option as just propaganda, the opening of US strategy for Geneva,'' said thin, intense Luc de Smet, who works with Pax Christi in Flanders and the International Peace Information Service (IPIS) in Antwerp.
''It's good that Reagan is showing flexibility for the first time and that both sides are willing to talk. But Geneva is limited to land-based missiles only, instead of covering those in the air and at sea as well. France and the United Kingdom should also be included in the talks.''
Laurens Hogebrink, an Amsterdam writer and organizer with the Netherlands Reformed Church, represents the strong Northern European Protestant strand in the peace movement.
''Four-hundred-thousand people spent all day in Amsterdam Nov. 21,'' he said. ''We expected half that number. The previous record demonstration in Holland had been 80,000. . . .
''The Geneva talks are part of the old strategy: diplomatic gambits, opening positions, talks and more talks. . . . But hundreds of thousands of people are demonstrating now for radical new policies.
''Each side should reduce its arms. We don't expect anything from Geneva. NATO is at a crossroads. If NATO keeps building arms, the Soviets will do the same. If NATO starts cutting, the Soviets will be under pressure to cut. We could use economic policy to encourage them. . . .''
Bruce Kent of CND in London: ''Bargaining positions in Geneva are irrelevant to us. The statement that you can only bargain from strength or parity is the very fuel of the arms race.
''One US Polaris missile carries enough warheads to destroy every Soviet city of any size. So the US could reduce arms without endangering its security.
''Of course, peace moves might not work. They carry risk. But they are worth trying. Can they be worse than a grotesque arms race that ends in nuclear catastrophe?''
Kent maintains that Britain's defense budget is far too high at a time of recession. '''Every British person spends (STR)4 ($8) a week for defense,'' he says. ''That's about (STR)16 ($32) a week per family. And 3 million British people don't have jobs at all. . . .''
Comments Dr. Gregory Treverton, associate director of the International Institute of Strategic Studies in London: ''The peace groups have been pretty effective. We've not seen their peak yet. When they claim the new cruise and Pershing missiles would turn Europe into the battleground, and that limited nuclear war is now more possible, the words have a wider resonance in Europe. . . .''
The strongest tribute to the impact of peace groups so far came from one pro-missile analyst in London who remarked: ''I've begun to worry whther the military advantages of the cruises and Pershings might not be outweighed by the political costs. . . . The sky won't fall if we don't deploy these 572 missiles. We could put them at sea and take other steps, I suppose. . . .''
The peace movement is growing in Germany, it is agreed, but is being countered by Schmidt's effectiveness as an intermediary with the Soviets and a blunting (perhaps temporary) of the anti-American edge of previous rallies.