Soviet leader Mikhail Gorbachev sought to win French support for a ban on space weapons today, at the start of his first official visit to the West since assuming power in the Kremlin.

Speaking at a state banquet in his honor this evening, Gorbachev made clear that he intends to make opposition to President Reagan's Strategic Defense Initiative (SDI) a major theme of his four-day visit. He denounced "an attempt to transfer military rivalry into space, as if there wasn't enough of it on earth."

"In the event that the instigators of this enterprise insist on continuing the dangerous course upon which they have embarked, the world will find itself facing hard times," he warned.

In an apparent attempt to stress that the Soviet Union can develop fruitful ties with Western Europe even if Soviet-U.S. relations remain strained, Gorbachev also spoke glowingly of the detente era in the 1970s and called for a strengthening of Europe's political role.

French officials and western diplomats viewed Gorbachev's visit as part of carefully planned public relations by the Kremlin leading up to the U.S.-Soviet summit next month in Geneva.

The Kremlin chief noted that both the Soviet Union and France wanted space to be used for peaceful purposes rather than "military confrontation." He said the Soviets would make every effort to prevent an extension of the arms race into space.

Although French President Francois Mitterrand has expressed strong reservations about SDI as a long-term threat to the present system of nuclear deterrence, he apparently has sought to avoid being used in a Soviet propaganda campaign against the United States. Officials here said he would emphasize his differences with Soviet policies both in public and in private.

Mitterrand, in his dinner toast, urged both superpowers to find a "reasonable compromise" in the Geneva arms talks and reaffirmed French opposition to the militarization of space. He refrained from direct criticism of the Soviet Union on human rights but spoke of the need to implement the 1975 Helsinki declaration that includes provisions on easier emigration and greater respect for fundamental freedoms.

The importance attached by Gorbachev to criticizing "Star Wars," as SDI is popularly known, was underscored when he raised the subject at airport arrival ceremonies, calling for "a return to detente, and the prevention of an arms race in space and its ending on earth."

The two leaders, flanked by their wives, stood side by side on the tarmac of Orly airport, to the strains of the "Marseillaise" and the Soviet national anthem. After the brief arrival statements, they then rode into Paris in a 50-car convoy for the first of three rounds of official talks at the Elysee presidential palace.

Gorbachev was given full honors due to a head of state even though his official position is general secretary of the Soviet Communist Party, a post he assumed on March 11 following the death of Konstantin Chernenko. The Champs-Elysees, the broad avenue that leads up to the Arc de Triomphe, had been decorated with red flags and the French tricolor in honor of the visit.

In interviews and statements before the trip, Gorbachev sought to warn western public opinion of dangers in a new spiral in the arms race, while at the same time conveying an impression of flexibility. He has tied Soviet proposals for deep reductions in stockpiles of strategic nuclear weapons to major restrictions on SDI.

The first round of private talks between Gorbachev and Mitterrand concentrated on East-West issues and detente, according to spokesmen for both sides, and lasted 135 minutes. The two men were accompanied only by interpreters and note-takers.

French presidential spokesman Michel Vauzelle said the two leaders discussed the balance of forces, particularly in Europe, and the need for procedures that would result in a "serious, verifiable reduction in the level of armaments."

In his toast, Gorbachev recalled that France and the Soviet Union were involved in the early days of detente during the '70s. He described the documents signed during this period, including the Helsinki final act dealing with human rights, as of major importance.

"The Soviet Union wants to see Europe -- the homeland of the Helsinki final act and the policy of detente -- increase its role in helping to improve the international climate. Both the Soviet Union and France, by themselves and sometimes, perhaps, together, can contribute to this," he said.

The French president is under domestic pressure to take a tough line with Gorbachev on human rights. The Soviet Union probably has a more negative image in France than in any other West European country because of the disillusion of former left-wing intellectuals with communism and public revulsion at the suppression of dissidents and the invasion of Afghanistan.

In anticipation of Gorbachev's visit, stickers with such slogans as "Gulag, no thank you" and "Gorbachev Gulag" have been plastered all over Paris. Several human rights groups are holding meetings to protest the visit, despite a ban on public demonstrations.

About 4,000 policemen have been mobilized to protect the Soviet leader, whose program includes conversations with French opposition leaders, a visit to a car factory outside Paris, and the obligatory pilgrimage to the Lenin museum. A separate program has been arranged for his wife, Raissa, including a visit with designer Yves St. Laurent.

Gorbachev is the first Soviet leader to visit France since the late Leonid Brezhnev in 1977. Traditionally warm Franco-Soviet relations deteriorated after Mitterrand's election in 1981 because of outspoken French support for the deployment of U.S. cruise and Pershing II missiles in Western Europe.

The Kremlin leader was accompanied from Moscow by Foreign Minister Eduard Shevardnadze, Deputy Prime Minister Ivan Arkhipov, and Deputy Trade Minister Nikolai Komarov.