U.S. Defense Secretary Casper W. Weinberger warned his NATO colleagues today that the "fragile" pro-defense constituency in the United States could fall apart if the Western Europeans failed to do their part by paying a fair share of the allied defense bill and by allowing new U.S.-made medium range nuclear missiles to be based on their soil.

"To safely move through the next decade," Weinberger told the NATO defense ministers at a meeting of the Alliance's nuclear planning group, "we cannot have another decade like the one just passed as far as the military balance is concerned."

It was the new U.S. defense secretary's first speech to a full-dress session of his Western Alliance counter-parts. To bolster his address, he flashed top secret satellite photographs showing what U.S. officials later said was the Soviet build-up in Poland and new weaponry now available to the Soviets.

From the Europeans, Weinberger heard responses that emphasized that the United States had cut back on defense spending in the past decade. They urged him once again to recognize the political problems Europe faces in going ahead with deployment of the new theater nuclear force in the absence of any form of arms control negotiations with the Soviet Union. Both Weinberger and Pentagon deputies traveling with him avoided naming a date to resume negotiations with the Soviets on reducing the number of European-based nuclear weapons.

They did reaffirm the general U.S. commitment to pursue such talks, however.

West Germany has been at the forefront of Western nations pressing for resuming those negotiations to offset growing antinuclear sentiment in Europe at the same time as alliance plans move forward to modernize NATO's European nuclear arsenal by adding Tomahawk land-based cruise missiles and the Pershing II battlefield rocket.

West German Chancellor Helmut Schmidt said in a published newspaper interview here today that he would be in trouble politically if the arms limitations talks about these weapons did not start this year. He added that difficulties would also arise in West German relations with the United States.

Reflecting the political problem, a crowd of 400 antiwar demonstrators today briefly blocked the entrance of the West German Defense Ministry compound where the ministers were meeting while Weinberger made his plea for higher defense spending.

The protesters, mostly women, sang, burned paper bombs and strung effigies of dead babies on a blue clothesline held across the ministry's tightly guarded gate during their noisy but peaceful demonstration this morning.

The NATO defense ministers seemed impressed by the slide show Weinberger put on.

Dutch Defense Minister Pieter de Geus told reporters after viewing the satellite photos and other top secret intelligence revealed by Weinberger that the Soviets have put 23 divisions in high state of readiness, two inside Poland and the other 21 in East Germany, Czechoslovakia and the western Soviet Union.

When the U.S. intelligence briefing moved from Poland to the Soviet strategic buildup, a picture of the huge Typhoon missile submarine was shown to underscore Weinberger's pleas for higher defense spending by NATO countries.

The Soviets, he told defense ministers, "find it administratively easy to play songs of accommodation abroad while marching to martial music at home."

President Reagan has responded to the Soviets' lack of accommodation by adding $32.6 billion to the fiscal 1981 and 1982 defense budgets inherited from former president Jimmy Carter, Weinberger said.

"These increases in strength do not come cheaply," he added, "because they require deep cuts in social programs which have political constituencies. The Amercian people will not want to march alone" to build up NATO defenses.

"If our effort is not joined by all who are threatened, by all who face the common danger, we in the United States could lose at home the critical public support for which we have labored long and hard."

[In Prague, Soviet President Leonid Brezhnev repeated in a speech to the Czechoslovak Communist Party the Soviet offer of a moratorium on "further deployment of new, and replacement of both Soviet and NATO medium-range nuclear-missiles stationed in Europe." He placed new emphasis on the Soviet willingness to consider the moratorium as a temporary step that would create "a more favorable atmosphere for talk" on the possible reduction of existing theater nuclear forces.]

[The United States and Western Europe have rejected Brezhnev's offer as a propaganda ploy intended to keep Soviet missiles already targeted on Western Europe in place while forestalling the planned Pershing and cruise deployment.]