Defense ministers of 11 North Atlantic Treaty Organization nations reiterated their "full support" today for President Reagan's proposal to eliminate all Soviet and U.S. medium-range missiles in Europe and rejected Soviet President Leonid I. Brezhnev's most recent proposal for a "moratorium" on new missile deployments.

The ministers, attending a two-day session of NATO's nuclear planning group, noted again that the Soviet proposal would leave 300 new Soviet SS20 missiles in place while NATO would be prevented from deploying, beginning late next year, about 572 new U.S. missiles designed to counter Soviet forces already in the field.

In a final communique at the close of the meeting, the ministers also noted that under the Brezhnev proposal Moscow could continue its buildup of SS20s east of the Ural Mountains, within range of NATO targets.

At a news conference, NATO Secretary General Joseph Luns said that, in December, 1979, when NATO first agreed on eventual deployment of new missiles and offered to negotiate reductions with Moscow, "Brezhnev declared . . . that a rough equality" existed in the number of nuclear weapons on each side in Europe.

Since then, Luns said, the Soviets have deployed 200 more SS20s and "still Brezhnev claims there is a rough equivalence. That must be a Russian phrase that we do not understand."

The communique also called attention to the new SS22 and SS23 missiles that Moscow is developing or deploying to replace older missiles.

Even though both are of shorter range than the SS20, the communique said the SS22 has enough range "to cover a substantial portion of NATO Europe" from its bases in the Soviet Union. The newer SS23 under development could hit Western Europe if it is moved into Eastern Europe, the communique indicated.

Defense Secretary Caspar W. Weinberger, asked about mounting protests in the United States about nuclear weapons, said:

"I think it is very important that people do realize the extent and nature of this Soviet threat. I don't think there is very much appreciation among some members of the public whose desire for peace is shared by everyone, whose aversion to nuclear war is shared by everyone but who believe, as we do not believe, that unilateral or some other form of disarmament without adequate balancing factors can produce peace. That's where we differ."

Although the NATO ministers' reaffirmation of support for the U.S. position and rejection of the Brezhnev proposal was expected, today's communique is the first formal response of the Atlantic alliance since Brezhnev's major speech on the subject last week.

At a news conference, Weinberger seemed to take in stride Tuesday's Senate subcommittee action making sizable cuts in the Pentagon's planned MX missile program.

Weinberger said that "we all would like" to have the MX deployed in a permanent and survivable way as soon as possible and that administration proposals on how to do this are best.

Weinberger left here on his first official Asian visit, a 12-day trip, with stops in Japan, South Korea and the Philippines scheduled.

The main event on his itinerary is the annual security consultative meeting with his South Korean counterpart, but he will also meet with top officials in each country and visit American troops and bases