National security affairs adviser Robert C. McFarlane said yesterday that the United States and the Soviet Union "are at a real moment of opportunity in arms control" and have the chance to achieve an interim agreement that would limit intermediate-range ballistic missiles in Europe.

Reviewing Reagan-era foreign policy in the first speech since he announced his resignation last week, McFarlane was optimistic about possible progress on arms control in the wake of the Geneva summit.

But he was pessimistic about prospects for Soviet withdrawal from Afghanistan.

In a speech to the World Affairs Council, McFarlane took note of the joint statement made by President Reagan and Soviet leader Mikhail Gorbachev at the end of the summit, where both leaders pledged to seek "early progress" in arms-control negotiations, particularly "in areas where there is common ground."

"The meaning of that phrase 'common ground' should be perfectly clear," McFarlane said. "It refers to those issues on which the long spadework of the past has paid off, above all to our agreement that there should be 50 percent reductions, appropriately applied in offensive strategic nuclear weapons. It refers to the possibility of an interim agreement on INF [the intermediate-range Soviet and U.S. nuclear missiles in Europe.]"

U.S and Soviet arms-control negotiators reconvene in Geneva next month. Progress on an INF agreement could provide a test of whether the summit accomplished anything more significant than several minor agreements and the easing of superpower tensions.

Recalling Winston Churchill's phrase after the World War II battle of El Alamein that it was "the end of the beginning," McFarlane said that this might one day be the description of the Geneva summit, first meeting of the superpower leaders since the Soviet invasion of Afghanistan in 1979.

"It may be, of course, that the Soviets will not live up to the words of the joint statement," McFarlane said. "But it may be that, ever so slowly, the ground is starting to move."

However, McFarlane said that post-summit statements by the Soviets about Afghanistan have been "discouraging." He said that at the summit Gorbachev took a "business-like, reasonable-sounding tone on the subject of Afghanistan," prompting U.S. policy makers to think that a negotiated withdrawal might be possible. But in a Nov. 27 speech Gorbachev restated the familiar Soviet position that U.S. support for the Afghan resistance is the reason for the continuing conflict.

McFarlane will not officially leave his White House post until the end of the year but has turned over the daily duties of his job to his successor, John M. Poindexter. White House officials said that McFarlane's speech last night was intended as a farewell address in which he reviewed, explained and defended the foreign policies of the Reagan presidency.

His principal contention, one often sounded by the president, was that the U.S. military buildup of the past five years and the restoration of "our sense of purpose and self-confidence" had created a realistic relationship between the United States and the Soviet Union.

"There are new opportunities before us now not because the president is changing his approach, but precisely because he isn't changing it," McFarlane said.

However, the departing national security affairs adviser made no mention in his 25-minute address of Reagan's favorite proposal, the missile defense plan formally known as the Strategic Defense Initiative and often called "Star Wars."

At the last round of arms-control negotiations in Geneva, the Soviets opposed any agreements on limiting offensive nuclear weapons, whether strategic or intermediate range, unless the United States would limit its missile defense programs to research instead of the testing and development contemplated by Reagan.

The phrase "interim agreement" on intermediate-range missiles cited by McFarlane was used in the Geneva joint statement at the insistence of the Soviets, who have made the curbing of Star Wars a condition to progress on arms control.