Georgi Arbatov, one of the Kremlin's leading America-watchers, said yesterday that the United States and the Soviet Union must "use the moment" of the forthcoming summit conference to improve their relations or "lose this opportunity for a very long time."

The director of Moscow's Institute of the U.S.A. and Canada, in an interview with editors and reporters from The Washington Post, said arms control is the area that holds the most promise for accord between President Reagan and Soviet leader Mikhail Gorbachev in their Nov. 19-20 meeting in Geneva.

In an interview with Time magazine last Wednesday in the Kremlin -- where Arbatov was seated next to the Soviet leader -- Gorbachev complained of a "campaign of hatred" and "scenario of pressure" by the Reagan administration leading up to the summit. At the same time, Gorbachev said the Soviet Union is "making very serious preparations" for the meeting with Reagan and "we shall be prepared to submit some very serious proposals."

Arbatov, in his comments here, reiterated both the strong Kremlin concern about recent U.S. actions and statements, and Moscow's readiness to make major proposals, especially in the arms field.

An Aug. 19 speech by White House national security affairs adviser Robert C. McFarlane, specifically addressed to the Kremlin, was read in Moscow as "absolutely negative," according to Arbatov. The speech "shows just how to ruin the relations" between the two countries, he said.

Arbatov said that there has been "a serious deterioration" of Soviet-American relations in recent weeks because of the McFarlane speech, the U.S. announcement of an antisatellite weapons test and U.S. charges that Soviet secret police had used "tracking chemicals" against U.S. diplomats.

Arbatov attributed the developments to concern in Reagan administration circles that Soviet initiatives, such as a moratorium on nuclear weapons tests, "might interfere with the 'empire of evil' concept on which the arms race is built."

At one point, Arbatov charged that the United States was exhibiting fascist tendencies. "Some very extremist factions have such a hold on your policy that I didn't expect 10 years ago," he said.

Arbatov would give no details of Soviet proposals, which, he said, are being discussed in Moscow before the summit. Some of them, he suggested, might be proposed to Washington through diplomatic channels before the November meeting in order to improve chances for a meeting of minds in Geneva.

The proposals now on the table in the Geneva arms talks and other arms control meetings are "not the last word," Arbatov said. Moreover, Moscow may make proposals in fields which have not been covered until now, he said.

For example,, Arbatov said it was his personal view that there could be a fresh approach to easing the dangers of tactical nuclear weapons deployed by the United States and the Soviet Union in Europe. These weapons have been discussed "only indirectly" in earlier arms talks, he said.

"We have proposed serious things" through announcements or in confidential negotiations with the United States and "we are ready to go forward," Arbatov said.

Arbatov said he plans to spend 20 days in the United States participating in public and private meetings and gathering impressions to take home to Moscow.