In the last allied consultations before U.S. and Soviet leaders meet in Geneva next month, NATO defense ministers today gave their endorsement to American complaints that Moscow was violating existing arms control treaties and were shown satellite photographs of newly deployed Soviet SS25 missiles.

The North Atlantic Treaty Organization allies received a three-hour briefing from Secretary of Defense Caspar W. Weinberger and other U.S. officials who described in graphic detail some of the alleged Soviet transgressions of the 1972 Anti-Ballistic Missile treaty and the 1979 Strategic Arms Limitation treaty.

Several European ministers remarked later that any lingering skepticism about the extent of Soviet violations had been removed. But they stressed that such evidence should not be exploited to cast doubt on the value of pursuing future arms control accords.

The mood of allied consensus was enhanced by a general wish to project a firm display of alliance solidarity in the weeks before President Reagan holds eight hours of meetings in Geneva with Soviet leader Mikhail Gorbachev on Nov. 19 and 20.

Weinberger illustrated his arguments today with up-to-date satellite photographs that portrayed three installations where the Soviet Union has purportedly sited the mobile, intercontinental SS25 missile. The United States claims the SS25 deployment breaks a SALT II treaty provision allowing the introduction of only one new strategic missile, which the Soviet Union claimed would be the SS24.

Moscow contends that the SS25 is merely a modified version of the antiquated SS13 missile, and that such modernization, in addition to deployment of one new missile, is allowed under the SALT II agreement. But the photographs displayed by Weinberger, according to ministers who attended today, indicated that the SS25 is a radically different missile.

West Germany's Manfred Woerner said the reconnaissance photographs showed the SS25 to be 10 percent longer, 11 percent wider and possessing 92 percent more throw weight, or explosive power, than the SS13. Under terms of SALT II, a modernized missile should not exceed its predecessor's length, diameter, launch weight and throw weight by more than 5 percent.

On Oct. 23 at a gathering of the Soviet-led Warsaw Pact nations in Bulgaria, Soviet spokesman Vladimir Lomeiko described Weinberger's initial claim about SS25 deployment as an invention that "does not correspond to reality." But he stopped short of an outright denial that the missile had been deployed and focused instead on denying it was a new missile that violated the SALT agreement. "Not one missile has been deployed which can disturb SALT II," he said.

Weinberger said the Soviets were also continuing to thwart American efforts to monitor their missile program developments by encrypting test signals, considered another violantion of SALT II.

The United States also gave what several participants called a "very elaborate explanation" of the Soviet Union's efforts in strategic defense as a rebuttal to those who oppose further development of Reagan's Strategic Defense Initiative.

The presentation included a scale model of the controversial phased array radar system near Krasnoyarsk that some Reagan administration officials believe will fulfill early warning tasks -- also forbidden by the ABM treaty -- as an integral part of a nationwide missile defense system.

"What we were shown convinces me that the treaty violations are not to be disputed," Woerner said, but he added that neither the United States nor the alliance should draw the conclusion that arms control agreements should be aborted because of the Soviet infringements.

Other European ministers echoed Woerner's assessment. Norway's Anders Sjaastad said he was dubious before the meeting about the dimensions of Soviet violations "but the material presented today was very convincing and left no doubts in my mind."

Weinberger is due to give Reagan a long-awaited report in two weeks outlining the nature of Soviet violations and a list of possible American responses. The special study is expected to be unveiled only days before Reagan meets Gorbachev.

The timing has generated some apprehension in Europe because of a feeling that the report's predictably negative conclusions could harm prospects for a successful summit.

But European ministers emphasized today that the allies wish to set aside any differences in the interests of allied harmony before the summit.

They did not wish to ignite a new confrontation with the United States over the gravity of the violations and the likely consequences for the future of arms control, delegates said.