A flurry of visits to Soviet Bloc capitals has provided leaders of key European Community countries with glimpses into the closed world of Kremlin politics and an opportunity to exchange notes on the bleak state of East-West relations.

The opening session of a two-day Western European summit meeting was devoted today to an examination of prospects for dialogue following an official visit to the Soviet Union by President Francois Mitterrand of France last week. French officials said Mitterrand returned from Moscow more than ever convinced of the need for firmness in dealing with the Kremlin.

With scarcely any real dialogue going on between the two superpowers, the Western Europeans have been taking the lead in putting out feelers to Moscow. The Soviet leaders have in turn used the visits to try to play the Western partners off against each other and put on record their view that improvements in East-West relations depend on a fundamental change of heart in Washington over arms control.

In addition to Mitterrand, recent Western European visitors to the Soviet Union have included foreign ministers Hans-Dietrich Genscher of West Germany and Giulio Andreotti of Italy. Geoffrey Howe, the British foreign secretary, will visit Moscow early next month. Both Britain's Prime Minister Margaret Thatcher and West Germany's Chancellor Helmut Kohl have recently returned from trips to Hungary, the Soviet Bloc country with the most developed ties to the West.

Mitterrand's official spokesman, Michel Vauzelle, said that the French president shared his impressions of Kremlin leaders with other heads of government and was congratulated by them on the firm line he took in Moscow.

The French leader's outspokenness in Moscow about sensitive human rights questions, such as the plight of dissident Soviet physicist Andrei Sakharov, has won wide praise in Western Europe. It is unlikely, however, that his comments will have much impact within the Soviet Union, where they were censored in the official mass media.

Conclusions reached by Western European leaders about present policy in Moscow have varied according to their national point of view and the treatment they received from the Soviets. Mitterrand's aides said the tone of Kremlin leaders in discussions with the French president appears to have been more restrained than the tirades to which the foreign ministers of West Germany and Italy were subjected.

When Genscher visited the Kremlin last month, his aides were struck by the increase in the authority of Andrei Gromyko, the veteran Soviet foreign minister. They described how Gromyko often interrupted the new Soviet leader, Konstantin Chernenko, on matters of substance and took over the presentation from his 72-year-old boss, who they said appeared stiff and weak.

A slightly different picture of the Kremlin power structure was painted by French officials. While conceding that Gromyko appeared to be playing an increasingly important role in the formulation of foreign policy because of his experience, they described reports of Chernenko's physical and intellectual incapacity as exaggerated.

At a private session with Mitterrand Thursday, according to French officials, the Soviet leader engaged in debate and developed coherent arguments in addition to reading from a set brief.

They said that in public appearances, Chernenko had trouble walking more than a few steps without help and seemed constantly out of breath, while at a gala evening of Russian opera at Moscow's Bolshoi theater in Mitterrand's honor, the Soviet leader excused himself after 15 minutes, saying that he was tired.

The French guests had a chance to study the personalities of the Soviet leaders Thursday at the formal Kremlin banquet, which was attended by seven of the 12 members of the ruling Politburo. They were particularly impressed by the intellectual vigor of Mikhail Gorbachev, who at 53 is the youngest member of the Politburo and a possible candidate to succeed Chernenko.

When Mitterrand mentioned Sakharov's name in his formal toast at the banquet, the French officials said, the jaw of Soviet Defense Minister Dmitri Ustinov dropped and Chernenko gave a little jump. The French leader had to urge the official interpreter to translate his words into Russian.

French officials said that the incident did not noticeably affect the atmosphere Friday at the rest of the talks, which they described as relaxed. The mood in the official French party later was one of self-congratulation, with Mitterrand's aides claiming that he had set a precedent by raising subjects in the Kremlin that "no other Western leader dares mention."

The Soviet reaction to Mitterrand, as reflected in press commentaries and private comments by Kremlin officials, appears to have been a mixture of irritation at his outspokenness and a desire to encourage France's traditionally maverick role within the Western alliance.

The French president was rebuked by Kremlin spokesman Leonid Zamyatin for his remarks about Sakharov, who declared a hunger strike May 2 in support of his wife's request for medical treatment in the West. But Soviet officials also expressed appreciation of Mitterrand's criticism of U.S. policy in Central America and support for a treaty banning the use of weapons in space.

In a cautious commentary on the visit, the official Communist daily Pravda said that "Washington had not succeeded in depriving its allies totally of their independence and their aspirations to consolidate their ties with the Soviet Union."

A Soviet journalist contrasted the Mitterrand visit favorably with Genscher's trip the month before, which he said had yielded nothing positive.

In ceremonies at Stalingrad (now Volgograd), Mitterrand pointedly ignored Soviet warnings about German revanchism and called for reconciliation between the victors and vanquished of World War II. He also reacted coolly to Soviet calls for a resumption in the practice, established under his predecessors, of annual Franco-Soviet summit meetings.

The tone of French press comment on the visit was set by the Paris daily Le Monde, which said that Mitterrand had managed "to avoid most of the numerous traps that threatened his initiative and win the challenge that consisted of speaking the same language in Moscow as in Paris or Washington."