Declaring that he had been "somewhat encouraged" by a "serious response" from the White House to the package of Soviet arms control proposals presented last week, Soviet leader Mikhail Gorbachev today used his first western press conference to set a relatively positive tone for his meeting with President Reagan in Geneva next month.

"It was not the usual 'no' " from Washington, Gorbachev told several hundred journalists in an almost two-hour joint press conference with French President Francois Mitterrand at the Elysee Palace.

But Gorbachev received an immediate setback for the new proposals he unveiled here yesterday when Mitterrand said France would not agree to direct negotiations on its nuclear forces with the Soviet Union. Mitterrand urged the Soviet leader to continue the current negotiations in Geneva on arms reductions with the United States.

Gorbachev had proposed that France and Britain open direct talks with the Soviet Union on reducing medium-range missiles in Europe. Britain also indicated today it was not ready to accept the Soviet offer to negotiate. Details, Page A23.

The 54-year-old Gorbachev coupled his repeated calls today for a renewal of the spirit of detente with continuing sharp criticism of President Reagan's Strategic Defense Initiative, or the so-called "Star Wars" research project, which is aimed at investigating future space-based antimissile defenses. The Russian said this must be halted if the two superpowers are to make any progress in negotiating offensive arms reductions.

On this point, the Soviet leader appeared to pick up some support from Mitterrand, who warned that there could be no substantial reduction in offensive long-range missiles and bombers by the superpowers unless the "transferring of the arms race from earth to space" is halted.

Mitterrand, however, separated his own well-known views about the U.S. SDI project from the Soviet attacks on Reagan's plan and repeatedly termed France and the United States as loyal allies.

Gorbachev balanced his admonition against Star Wars with a judgment that last week's New York talks between U.S. Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze had given him "an impression of promise" for the Geneva summit.

Asked about the prospects for failure at the Geneva meeting, Gorbachev replied: "Let us not prejudge the summit."

At another point, he added, "We have decided to add complementary proposals to give impetus to the Geneva negotiations," apparently signaling that the Soviet Union's strategy of appealing to western countries with a series of arms-related plans will continue up to the summit, scheduled for Nov. 19-20.

In addition to the package of arms control proposals outlined to U.S. leaders last week and presented before members of the French parliament yesterday, the Soviet leader has proposed a joint ban on testing of nuclear weapons and a moratorium on deployment of intermediate-range weapons.

In this rare press conference by a Soviet leader, Gorbachev received questions with the help of translators and responded with little reference to notes.

Arms control continued to dominate the public and private exchanges between Gorbachev and Mitterrand, who concluded three days of talks today. The Soviet leader returns to Moscow Saturday.

Seated a few inches from Gorbachev, Mitterrand based his stiff rejection of the Soviet proposal for missile talks on his contention that the French force is too small to permit him to negotiate any of it away and that France must retain the ability to deter an attack. "France must stay above the threshold of credibility," he argued.

Mitterrand then softened his rejection by saying that France is always prepared for an "exchange of views" with the Russians on the subject of its nuclear forces, which are about one-seventh the size of U.S. and Soviet nuclear forces.

In response to Mitterrand's rejection, Gorbachev quickly recast his proposal during the press conference. The Soviet leader said he would favor starting "an exchange of views which might lead us into negotiations." Gorbachev added that he thought "the French leadership felt the constructive thrust of our proposals."

Gorbachev's proposal would take the SS20 missile force out of the current Geneva negotiations with the United States and have the SS20s counted against French and British launchers and bombers instead of against American medium-range missiles in Europe.

In comments to journalists before the press conference, Soviet negotiator Yuli Kvitsinsky said that the Russians would continue talking about medium-range missiles in Geneva with the United States while trying to get discussions started with the French and British.

The Soviet and American teams dealing with medium-range missiles met for three hours in Geneva today. Talks will be held on a normal schedule next week.

Gorbachev fielded a range of questions, from Soviet vulnerability in Lebanon to the fate of dissident physicist Andrei Sakharov, exiled in the Soviet city of Gorki. His performance represented a considerable departure from the brittle language and hardened positions often associated with his predecessors.

Harkening back to the East-West detente that peaked in the early 1970s, Gorbachev called for "the need to do everything possible" to move back toward that era.

"Detente is not just in the spirit of recollection of the past," he said, but "a lesson we could draw from."

The Soviet leader said that the case of Sakharov has been "submitted to competent authorities for consideration," an advance from the previous Soviet insistence that the physicist, banished from Moscow in 1980, could not be allowed to leave the Soviet Union because he possesses state secrets.

The Soviet leader tightened the pressure on U.S. leaders for an agreement on SDI by threatening that without such an agreement, "I don't know that we would be able to engage in negotiations at all."

Gorbachev also attacked the responses of some U.S. officials to the Soviet proposals. The initial U.S. response of "where are these radical proposals" has become "what are you pressuring us for," Gorbachev said.

He reiterated his announcement yesterday that the Soviet Union has reduced its deployments of SS20s directed at Europe to June 1984 levels by removing some from "standby alert" and said, "any allegations that we have carried them over to Asia are ungrounded."

Gorbachev said there are now 243 missiles deployed in the European part of the Soviet Union. U.S. intelligence sources have said that the Soviets have shifted about 27 of their deployed SS20s eastward.

But Gorbachev refused to answer a question about how the current overall level of SS20s compared to the June 1984 level, saying that the government of the Netherlands has been informed on the subject. The Dutch government has said it will approve a deployment of 48 NATO cruise missiles if Soviet deployments of SS20s are now higher than they were in June 1984. NATO officials say those levels are higher.

Gorbachev said that installations for the missiles that had been removed would be dismantled over the next few months and said the United States could confirm this through photographs.

Batting back questions on the controversal subjects of Jewish emigration from the U.S.S.R. and the number of Soviet political prisoners, he said he had already answered those questions. In an interview broadcast on French television Tuesday night, the Soviet leader claimed that Jews enjoy political and other rights in the Soviet Union and called assertions that there are 4 million Soviet political prisoners "absurd."

On the Middle East, he declined any substantive comment on the hostage drama in Beirut involving Soviet diplomats, saying only that terrorist tactics were "unacceptable" and "would accomplish nothing." He said the Soviet Union would resume diplomatic relations with Israel once the political situation in the region had been normalized and he reiterated the Soviet recognition of Israel's right to exist.

Although Mitterrand said he urged Gorbachev to move toward a settlement in the five-year-old Afghanistan conflict that would guarantee the Soviet-occupied country's neutrality, the Soviet leader avoided mentioning the country at all.