VENICE, JUNE 9 -- After six years of dominating the annual summit meetings organized by the world's leading industrial democracies, President Reagan has found himself on the defensive and unable to impose his agenda on the opening stages of this year's gathering in Venice.

The political and economic problems that confront almost all of his six fellow leaders have prevented any of them, however, from substituting a clear and coherent alternative view in a summit that thus far has lacked focus and drama.

The summit's opening dinner, which often sets the tone for the three-day event, last night found Reagan trying to dispel European concerns that his agreement to remove U.S. medium-range missiles from Europe will affect the Atlantic alliance's overall military strategy against the Soviet Union.

Significantly, it was British Prime Minister Margaret Thatcher, Reagan's closest ally here, who led the way in politely but clearly pressing the president to explain the impact he expects his negotiations with the Soviet Union to have on the North Atlantic Treaty Organization's "flexible response" strategy of deterring or combatting any type of Soviet attack on Western Europe.

The dinner conversation was thus not a battle over conflicting objectives, as has been the case at past summits when Reagan has pushed U.S. economic priorities or his Strategic Defense Initiative plan for a space-based antimissile defensive shield.

Instead, it was an examination by concerned allies, containing an undercurrent of skepticism about American leadership in Reagan's final 18 months of office, according to informed accounts of the 3 1/2-hour dinner discussion.

The session also indicated that new differences over allied strategy on future negotiations with the Soviets on battlefield nuclear weapons -- very short-range missiles and artillery not covered by the would-be superpower accord -- are emerging.

West German Chancellor Helmut Kohl, who remained silent for most of the discussion, has insisted that the West has to go through the motions of being ready to negotiate on battlefield nuclear weapons.

But he heard no echo of that position in the comments by Reagan and Thatcher, who appear to favor instead establishing a negotiating "firebreak" that would rule out the elimination of a category of nuclear arms that would be used only on German territory in a conflict involving NATO and the Warsaw Pact. Those who oppose further negotiations on these battlefield weapons believe they are necessary to help deter an attack by numerically superior Soviet ground forces.

Even the way in which the content of the discussion became known stood in revealing contrast to previous years, when American briefers have aggressively pushed the White House version of the president's accomplishments among his peers.

This year, the White House issued a laconic three-paragraph summary of the dinner after it became known that other delegations would brief journalists in some detail.

French President Francois Mitterrand, 70, was made so enthusiastic by the discussion that he took three of his aides on an hour-long walk along the canals and bridges of Venice immediately after the dinner to recount the conversation in detail to them. They then briefed a 2 a.m. press gathering.

Their account was confirmed in its essentials by British, West German and other delegates. Secretary of State George P. Shultz also acknowledged to reporters that the leaders had been willing to say to each other, "Yes, there are problems, there are changes," and to discuss them "in a direct, realistic and informed way."

The discussion cleared the way for the quick endorsement this morning of three political declarations -- on terrorism, the Iran-Iraq war and East-West relations. None of the statements broke new ground.

The East-West declaration reflected what European delegates said had been a thorough discussion at dinner last night of Soviet leader Mikhail Gorbachev's new policies.

Thatcher, who visited Moscow in March, took the lead and then began to draw Reagan out on his views on the impact that the tentative U.S.-Soviet intermediate-range nuclear forces (INF) agreement would have on NATO strategy.

Under the INF agreement, the superpowers would withdraw all nuclear missiles under their control in Europe with a range of more than 300 miles. That would leave NATO with an estimated 4,600 nuclear warheads in Europe, most of them on tactical battlefield missiles, artillery shells or mines.

For Thatcher, Kohl and other European leaders, the U.S. Pershing II and ground-launched cruise missiles that would be eliminated by the INF agreement formed a significant component of NATO's "flexible response" strategy, designed in the early 1960s to give the United States a variety of tactical nuclear responses to a Soviet attack in Europe short of a cataclysmic massive retaliation on Soviet territory from missiles based in the United States.

The INF agreement, now in draft form in negotiations in Geneva, would mark the first time that NATO weapons systems would be eliminated as a result of an arms control accord and has created serious misgivings within the NATO military establishment.

In response to Thatcher's question, which goes to the heart of allied concerns, Reagan reportedly only repeated for the other leaders portions of a televised speech he gave last Friday in which he reaffirmed the American commitment to defend Europe.

He also sought to reassure the group that flexible response would not be affected by removal of the medium-range missiles.

Mitterrand, noting that France was not affected by the discussion on flexible response because it does not belong to NATO's integrated military command, also reportedly called attention to Reagan's recently renewed commitment to a common strategy built around nuclear deterrence, which he suggested was the key element of maintaining peace. The traditional American commitment to use its long-range missile force as an umbrella over the allies as well had been called into question after the U.S.-Soviet summit in Iceland in October in which both leaders nearly agreed to get rid of all nuclear missiles.

Mitterrand clearly implied that nuclear weapons that could hit Soviet territory were the heart of a deterrence strategy while the battlefield variety were not really an important concern for alliance strategy.

But Kohl, in putting aside strong reservations about giving up the American INF rockets, has insisted that the INF agreement should be followed by talks between the superpowers on reducing or eliminating the short-range, battlefield nuclear missiles as well.