French President Francois Mitterrand, staying away from a postsummit briefing by President Reagan in Brussels, called today for cooperation among Western European countries to counter a possible extension of the arms race into space.

Addressing his first national press conference in 18 months, Mitterrand described the outcome of the Geneva summit as a "hopeful sign" for East-West relations. But he went on to distance France from the arms negotiations now under way between the two superpowers and reasserted the importance of maintaining an independent nuclear deterrent.

French political analysts depicted the two-hour press conference as an opening shot by Mitterrand in what is likely to be a grueling campaign for legislative elections next March. The latest opinion polls show the president's Socialist Party trailing the right-wing opposition.

Mitterrand's decision not to attend the special session of the North Atlantic Treaty Organization in Brussels marked the second time in less than a month that he has turned down an opportunity to meet with Reagan.

Last month he declined an invitation to a presummit meeting of the seven leading industrial democracies at the United Nations on Oct. 24.

Mitterrand was reported by French officials to have been angered by the fact that the invitation to the Oct. 24 meeting was publicized in Bonn and Tokyo before he had received it, and by the appearance of Reagan converting the industrial summit, which was originally a French initiative, into an allied show of support for American positions.

Moreover, the French leader also took personally the White House's decision not to accept a secret offer Mitterrand made to come to Washington last month to brief Reagan personally on the talks the French leader held with Gorbachev in Paris in October, according to French sources.

A senior French official said later that Mitterrand would have been happy to meet with Reagan for a substantive bilateral discussion but saw little point in attending "protocol" meetings such as today's NATO session in Brussels. France was represented by Foreign Minister Roland Dumas.

The aloofness displayed by Mitterrand to western summit meetings is generally popular in France, with its strong sense of national pride. The consensus on foreign and defense policies has provided the president with a rallying point that could stand him in good stead if the left loses next year's elections.

At today's press conference, Mitterrand said France should act to safeguard the credibility of its independent deterrent by taking measures to protect itself from an extension of the arms race into outer space. He said the development of space technology could only be effective if carried out in cooperation with other West European countries.

"If the two superpowers have the imprudence to get involved in 'Star Wars,' any country that wants to maintain its independence will have to equip itself with the means . . . to protect its territory," Mitterrand said.

France already has taken the lead in launching a civilian space research program, known as Eureka, with its European partners. Earlier this month, Defense Minister Paul Quiles said France should attempt to preserve the credibility of its nuclear strike force by developing miniaturized nuclear warheads that would be invisible to defensive radars.

Although Mitterrand called for greater cooperation between European countries in space research, he stopped well short of proposing a joint defense system for Western Europe. He noted significant barriers to European defense, including the present political taboo against allowing West Germany a finger on the nuclear button.

Mitterrand refused to discuss how he would react in the likely event of an opposition victory in next year's legislative election, insisting only that he would do his duty. He noted that his seven-year term is not due to expire until 1988.