Soviet leader Mikhail Gorbachev today publicly launched a new Soviet arms control initiative by proposing that the United States and the Soviet Union agree to cut their strategic missile forces by half and negotiate a total ban on the development and deployment of space-based weapons.

Unveiling the wide-ranging arms control proposals in a speech to French legislators, Gorbachev altered several previous Soviet negotiating positions in an apparently calculated bid to rally European public opinion behind him before he meets President Reagan in Geneva on Nov. 19 and 20.

In addition to confirming that the 50 percent cut in strategic weapons by the superpowers has now been proposed, the Gorbachev speech included these key points:

*He insisted that Moscow could "no longer ignore" the modernization of French and British nuclear arsenals and called for the first time on France and Britain to join the Soviet Union in direct arms talks. Previously the Soviets considered French and British missiles part of the overall western total in bilateral arms negotiations with the United States.

*He said Moscow believed it was possible to reach a separate accord on the reduction of medium-range missiles in Europe without the "direct connection with the problem of space and strategic arms." Until now the Soviets have insisted that all three elements of the Geneva arms negotiations -- involving strategic and intermediate-range missiles and space defense -- be integrally linked.

*Gorbachev disclosed that the Soviet Union had unilaterally "withdrawn" some of its SS20 medium-range missiles stationed "in the European zone" of the U.S.S.R. and targeted on Western Europe. He said this reduced the number to 243, the level that existed on June 1984. This action is seen here as a clear attempt to influence the decision to be made by the Dutch government Nov. 1 to approve or reject the stationing of 48 U.S. cruise missiles in the Netherlands.

*The Kremlin leader defined the term "strategic forces" in a way that includes U.S. medium-range Pershing and cruise missiles stationed in Europe but excludes Soviet SS20s with European targets. His formula calls into question the future of the Soviet-U.S. talks on medium-range missiles at Geneva that were previously linked, at Soviet insistence, with parallel talks on strategic and space weapons.

*While advancing no specific new proposals on space-based weapons systems, Gorbachev also appeared to interject a deliberate note of ambiguity into the strident Soviet attacks on President Reagan's Strategic Defense Initiative by omitting a specific call for a ban on research on space weapons.

This omission was repeated during a briefing given to reporters by his press spokesman, Leonid Zamyatin, and a senior member of the Soviet negotiating team in Geneva, Yuri Kvitsinsky. Kvitsinsky later said, in reply to a question, that the Soviet Union was prepared to permit "basic research" on space-based defense systems, echoing a position taken by Gorbachev in an interview with Time magazine last month.

Gorbachev said that the implementation of the Soviet proposals would result in a substantial step forward to the goal of "prohibition and total liquidation of nuclear arms, the total delivery of mankind from the threat of nuclear war."

"There can be no victors in a nuclear war," he told the legislators. "It is time to draw a practical conclusion from this: stop the arms race."

Although western experts were virtually unanimous in praising the skill with which Gorbachev presented the new Soviet proposals, there was much greater skepticism about the detailed content of the package.

Both French and British officials reacted coolly to the new Soviet leader's call for direct talks on theater nuclear weapons in Europe. In the past, both countries have refused to consider such negotiations on the grounds that their independent nuclear forces are insignificant compared to the very much larger arsenals of the two superpowers and are needed to deter attacks on their homelands.

French legislators who listened to Gorbachev's speech said they were struck by the new tone adopted by the Soviet leader but added that they wanted to examine the fine print of his proposals before making a judgment. Other analysts said he appeared to be trying to split the western camp and "decouple" the defense of Western Europe from that of the United States.

"Gorbachev's aim is to divide the Europeans. In France, the government can explain fairly easily to public opinion why it must reject the Soviet proposal for direct talks on its nuclear deterrent. But that will be much more difficult to explain to other Europeans, notably the West Germans," said Philippe Moreau Defarges of the French Institute for International Relations.

Western analysts said the Soviet definition of U.S. and Soviet strategic weapons -- weapons capable of "reaching the other side's territory" -- appeared to exclude the Soviet force of medium-range SS20s that are targeted on Western Europe. These weapons, the Soviet proposals implied, would be traded off against the 178 French and British missiles.

Noting French and British objections to the inclusion of their nuclear forces in the U.S.-Soviet arms talks in Geneva, Gorbachev said that it followed that "it is time to start between us a direct dialogue on this theme and try to find an acceptable way out through joint effort."

The French strategic triad is made up of missile-carrying nuclear submarines, bomber aircraft and ground-based missiles that are based on the Albion plain in southern France. Under a nuclear modernization program announced by the French government, the number of warheads carried by the French strategic submarine force will increase to 480 by the end of the century.

The British nuclear force is made up of four submarines, each of which carries 16 Polaris missiles. Britain is to modernize its Polaris system by the mid-1990s when it will introduce Trident missiles with 14 warheads each.

Gorbachev said that Soviet SS20 missiles deployed in retaliation for the deployment of U.S. "Euromissiles" had all been withdrawn from "standby alert." He added that "the stationary installations for housing these missiles will be dismantled within the next two months."

U.S. intelligence sources reported in Washington last month that they had detected a Soviet redeployment of 27 SS20 missiles out of the European zone in recent weeks. The sources noted that the SS20 bases appeared to be undergoing modifications that would allow them to be the launch sites for an equal number of SS25s, a new Soviet intercontinental ballistic missile.

In a direct attack on the Strategic Defense Initiative, popularly known as "Star Wars," Gorbachev said that the idea was "an illusion and an extremely dangerous one at that." Aides later said that the proposed Soviet ban on space-based weapons would cover "production, development and deployment."

"It is naive in general to search for a solution of the problem of security in the perfection of the shield and the sword," he added.

At NATO headquarters in Brussels, officials expressed disappointment that Gorbachev had decided to go public with Moscow's latest arms control proposals. They said the proper way to discuss such matters is in confidential negotiations in Geneva.

In an interview on French television earlier this week, Gorbachev complained that the United States was breaking the confidentiality rule that has been imposed on negotiators in Geneva. He said that there was a tendency for anything that happened in Geneva to be leaked "10 minutes later" to the outside world.

In his speech today, Gorbachev said that he was making the Soviet position public "because a multitude of versions and false rumors are already circulating in the West concerning our proposal and it is time for some clarification."

An indication of French interest in the new Soviet arms control initiative was provided by the scheduling of an extra round of one-on-one talks between Gorbachev and President Francois Mitterrand this afternoon. A plenary meeting between the two delegations was canceled.

French presidential spokesman Michel Vauzelle said the two leaders discussed arms control and "the balance of forces in the world and in Europe" during their second round of talks today.

Gorbachev gave no further public details of how the 50 percent cutback in ocean-spanning strategic missiles and bombers might be applied.

Gorbachev, who is making his first visit to a western country as Soviet Communist Party leader, will have a chance to expand on his arms control proposals at a joint news conference Friday with Mitterrand. It is believed to be the first time in more than 30 years that a Soviet leader has agreed to take part in a western-style press conference.

While Gorbachev has preferred to hammer away at the theme of avoiding nuclear confrontation in his meetings in Paris, he has been repeatedly criticized by French leaders for human rights violations in the Soviet Union.

The sharpest attacks so far have come from the mayor of Paris, Jacques Chirac, who heads the neo-Gaullist Rally for the Republic. In a welcome speech at city hall this morning, he accused the Soviet Union of violating the 1975 Helsinki declaration on security and cooperation.

"The nobler the hope, the more bitter is the disappointment. All the commitments of the Helsinki final act have not been kept, particularly those concerning human rights and freedoms. I think with emotion about the fate of those who are deprived of liberty because of their beliefs. I think also of Soviet Jews who are not allowed to leave the country," Chirac said.

Gorbachev, who was accompanied by his wife Raisa, nodded briefly as Chirac's remarks were translated into Russian. He did not attempt to respond in his prepared reply.

Chirac, whose party has favored close ties with Moscow, also asked why the Soviet Union felt it had to construct "in the face of Western Europe a conventional, chemical and nuclear arsenal which seems to us to be totally out of proportion with its own security needs and those of its allies."

French Prime Minister Laurent Fabius also raised human rights issues with Gorbachev at a meeting this morning, handing him a list of specific cases that France would like to resolve. The list included seven cases of French-Soviet marriages, as well as separated families and parents who are unable to visit their children because of travel restrictions.

Tonight the Soviet leader and his wife were entertained at a gala theater evening at Versailles, the residence of French monarchs until the revolution.