Bonn — In a closely guarded Bundestag chamber, the 16 NATO heads of state voted overwhelmingly to stay the course against military aggression from the East. And they resolutely backed President Reagan's arms talk approach to the Soviets.
At the same hour across the Rhine -- serving as a swift, formidable moat -- thousands of West German peace demonstrators gathered. They represented peace movements on both sides of the Atlantic calling for an arms freeze or a unilateral Western decision to renounce first use of tactical nuclear weapons in Europe.
The NATO commanders did not compare their parley with the demonstration two kilometers away. They contrasted it instead with last week's acrimonious economic summit at Versailles.
They emphasized that the North Atlantic Treaty Organization summit in Bonn June 10 revealed greater harmony and broader agreement than the alliance has generally shown in recent years.
A united Western military front was seen as critical to American negotiating strength with the Soviets on the three sets of arms talks under way or in prospect.
Washington won strong endorsement of all three of its arms negotiation positions: its START (strategic arms reduction talks) formula, announced May 9, which calls for talks with the Soviets to begn at the end of June; its ''zero option'' plan, announced Nov. 18, for Geneva-based medium-range missile talks; and Mr. Reagan's latest conventional arms reduction proposal, announced June 9 in the Bundestag, calling for a mutual drawdown to 900,000 troops in Europe on both sides.
Inclusion of Spain in the summit was also taken as a sign of a bolstered alliance. On May 30 Spain became the first new NATO member since West Germany joined in 1955. And the attendance of French President Mitterrand at the NATO chiefs' dinner, boycotted in the past by Mr. Mitterrand's predecessors, indicated a much more positive French attitude toward the alliance.
Demonstrations aside, Europe seems less agitated now about the process of arms talks.
It was less the physical arrival of President Reagan in Europe last week, however, than the advance news of his developing arms policies over recent months that has eased European tension, NATO officials say.
What the Europeans wanted was a better balance between the confrontation and cooperation in East-West relations.
The Europeans think the administration better understands their need for some trade and dialogue with the East bloc -- particularly the Germans, a divided people. Both the administration and Europeans seem satisfied for the moment with a compromise NATO call for ''genuine detente,'' a term each side can accent differently.
The Europeans are concerned now that Reagan might be satisfied with merely having put in place the machinery for talks, without pushing for results.
Public opinion in Europe reflects a wait-and-see mood as the Reagan administration and the Kremlin negotiate on the medium-range weapons, the Pershing II and cruise missiles, to be deployed on European soil if talks don't succeed by the end of 1983.
For the moment -- despite a potential anti-Reagan flareup in Berlin, -- Mr. Reagan may have helped contain the peace movements on both sides of the Atlantic , his aides claim.
In West Germany, the trend in public concern about peace had climbed by half - from about 50 percent of the public labeling it a ''most important'' concern in the mid-1970s, to 75 percent this spring.
Americans may be making too much of Reagan's impression on Europeans, officials here say.
''The question for Europeans is not Reagan's image -- whether he rides horses with the Queen of England -- but about his policies,'' says a West German official. ''Most people recognize Reagan can't afford to start negotiations just for show. He is under political pressure in America to show results before 1984, if he wants to seek reelection.''
For the time being however, Reagan leaves Europe having won an endorsement of his arms strategy at NATO and at least partial support at Versailles for his economic policy of making it tough for the Soviets to trade on credit. He was most measured in his remarks in the one country where it was most important, West Germany. His aides felt he could declare his 11-day mission a success.