President Bush and Soviet President Mikhail Gorbachev yesterday opened a summit to come to grips with a world they described as radically changed, and began to grapple immediately with the problem of a united Germany in a post-Cold War Europe.

Gorbachev, following four hours of White House meetings with Bush, said they had assigned their foreign ministers to begin more "in-depth discussions" of ideas about Germany offered by both sides during the meetings. However, it was unclear last night when the meetings would begin, and U.S. officials said there is no sign that Bush and Gorbachev narrowed their differences on Germany during the first day of talks.

A U.S. official said Gorbachev offered a plan to base future European security on the 35-nation Conference on Security and Cooperation in Europe, which would replace the NATO and Warsaw Pact alliances. The Soviet idea is to restructure this organization to safeguard European peace under a "Greater European Council" of all 35 heads of state, including those of the United States and Soviet Union, that would meet once a year.

Spanish Foreign Minister Francisco Fernandez Ordonez said yesterday he had received a letter from Soviet Foreign Minister Eduard Shevardnadze on Tuesday outlining this plan. U.S. officials said a copy of the Soviet plan was given to the State Department on Wednesday by Soviet Ambassador Alexander Bessmertnykh.

Shortly before the summit began, a senior adviser to Bush expressed skepticism about such a proposal, saying the 35-nation organization would be too unwieldy to guarantee the security of major nations. However, the official said Bush would propose an expanded political role for the group.

The U.S. ideas involve limitations on German and other military forces in the central zone of Europe as part of East-West negotiations on reductions in land armies in Europe. Officials said these ideas have been shared with top West German officials, and, in another sign of intensified diplomacy, Chancellor Helmut Kohl will meet Bush here next Friday.

A Soviet Foreign Ministry official said that such limits on German and other Western forces "are welcome" because they "show some movement" on the U.S. side, but that this is "clearly not enough" to convince Moscow that the unified Germany should be part of the NATO alliance.

Gorbachev kept up an energetic pace reminiscent of his first visit to Washington in December 1987. He jumped out of his limousine on 15th Street NW at New York and Pennsylvania avenues to talk with onlookers, and he spoke at length to a group of Americans invited to the Soviet Embassy for a luncheon. But, in contrast to his last visit, he seemed defensive in his luncheon remarks about his troubles at home.

Although the talks at the White House did not produce any breakthroughs, Gorbachev was upbeat last night in a toast at the state dinner in his honor. He suggested that the summit would exceed expectations and might produce "the biggest results compared to all the other" previous U.S.-Soviet summits. "Maybe I'm too optimistic," he added, "but let's wait and see. We have two days. I believe that maybe we will have those major results." Bush devoted his toast to a salute to Gorbachev "for the political and economic reforms you've introduced and for creating within the Soviet Union a commitment to change."

Emerging yesterday afternoon from the White House after his second two-hour meeting with Bush, Gorbachev provided the first substantial account of the discussions.

"I think it is not here that the German question will be resolved," he said through his interpreter, a statement seconded by Bush when he met reporters in the Rose Garden shortly after Gorbachev had departed.

Gorbachev said, however, that ideas from both sides on settling all parts of "the external aspects of German unification" gave rise to a decision to assign Secretary of State James A. Baker III, Shevardnadze and their respective experts to "in-depth" talks on "something that emerged that requires such discussion."

The idea of having Baker and Shevardnadze hold those discussions on Germany had originated with Gorbachev, Bush said, adding that he is "encouraged" by that. "I think the important point is we could talk very frankly . . . . Let's hope some of the differences have been narrowed," he said.

Although both Gorbachev and Bush indicated that these talks would begin immediately, there was confusion in the Bush administration about the public statements. A U.S. official last night quoted Baker as saying he had no plans for such talks on Friday. Baker and Shevardnadze are scheduled to meet in Copenhagen next week at a separate conference, and they will also be together at talks on the future of Germany in late June in East Berlin.

Bush said his "fundamental" position on the Germany-related issues is the same as when the discussions with Gorbachev started, suggesting that no major compromises of the U.S. posture regarding full-scale German participation in NATO is in the offing.

Earlier, Bush welcomed Gorbachev to the White House in a sun-drenched ceremony on the South Lawn, saluting the Soviet leader for his role in the momentous events of the last year. At the same time, Bush vowed in his official welcoming statement to confront Gorbachev directly on their differences over Lithuania and the outline of Europe.

"In a larger sense," Bush said, "the success of this summit depends not on the agreements we will sign, but on our efforts to lay the groundwork for overcoming decades of division and discord, to build a world of peace in freedom." Bush urged Gorbachev to join him "to further the process of building a new Europe, one in which every nation's security is strengthened and no nation is threatened."

Recalling the collapse of hard-line communist regimes in Eastern Europe and the breaching of the Berlin Wall, Bush said the world has witnessed "what just one short year ago seemed like a distant dream: a continent cruelly divided, East from West, has begun to heal with the dawn of self-determination and democracy.

"In Germany," he said, "where the Wall once stood, a nation moves toward unity, in peace and freedom."

Gorbachev, joining Bush on a platform with the Washington Monument and the Jefferson Memorial behind them, also paid tribute to the end of the Cold War. Since his last visit to the White House 29 months ago, Gorbachev said, "the world around us has also changed beyond recognition."

"The worlds, which for years separated the peoples, are collapsing," Gorbachev said. "The trenches of the Cold War are disappearing. The fog of prejudice, mistrust and animosity is vanishing."

While neither leader referred directly to their differences over whether a unified Germany should remain in the Western alliance, Gorbachev made a pointed reference in his statement to the high price the Soviet Union paid to crush Hitler's Nazi Germany 45 years ago. Fears of a revived, strong Germany have been at the heart of Soviet concerns about reunification.

Gorbachev said many Soviet people look on the improvement in U.S.-Soviet relations "with the hope that the tragedies of the 20th century, those horrible wars, will forever remain a thing of the past. I think this is what the Americans want too."

Bush, talking with reporters late in the day, said he was "not insensitive" to the 27 million Soviet lives lost in the war. He also recalled the heavy toll in American casualties, and said, "I reminded him that I was the only one of the two of us that was old enough to remember it from being there." Bush was a Navy pilot in the Pacific.

Remaining unresolved and apparently under discussion between the two sides was the question of whether a U.S.-Soviet trade agreement, the provisions of which have been agreed upon, will be signed during the summit. Soviet officials have displayed great interest in this pact, which could be a heartening symbol of future economic progress to the hard-hit Soviet consumers. But Bush has been reluctant to sign an economic accord while the Kremlin's economic embargo against the breakaway Lithuania continues.

Bush said last night he will discuss details of the trade issue with Gorbachev today.

White House spokesman Marlin Fitzwater said the two leaders or their aides will sign or announce a host of accords in the East Room of the White House at 5 p.m. today. The most important are a statement of agreement on the main points of a strategic arms reduction treaty (START) and a statement of objectives for a future START II pact featuring deeper cuts in nuclear arsenals.

A U.S.-Soviet accord to slash the chemical weapon stocks of both nations and require an immediate halt to U.S. production of additional chemical arms is to be signed, Fitzwater said. Also being signed are protocols, or detailed procedures, for implementation of the unratified 1974 and 1976 U.S.-Soviet nuclear testing treaties.

Among other agreements to be signed today, according to Fitzwater, are a new student-exchange agreement, a commercial aviation pact, a new cultural agreement, a maritime agreement and an ocean studies accord. Staff writers R. Jeffrey Smith and Ann Devroy contributed to this report.