The Soviet Union tonight sharply dismissed President Reagan's decision to continue compliance with the SALT II treaty by dismantling a U.S. missile-firing submarine as "a single measure . . . which does not change the overall picture" of U.S. attempts to undermine the arms control process.

The Soviets, in a statement issued by chief Foreign Ministry spokesman Vladimir Lomeiko, also accused the United States of using allegations of Soviet arms control violations to "crawl out" of the SALT II treaty, which was signed in 1979 but never ratified.

The Soviet accusations came after what appeared to be a day of long deliberations within the Kremlin over how to respond to Reagan's long-awaited decision concerning continued U.S. compliance with SALT II's provisions.

Earlier in the day, Lomeiko had declined comment on Reagan's announcement and said Moscow would reserve judgment until it had studied Reagan's statement.

The Kremlin's decision later in the day to attack Reagan's action appeared to surprise a number of western officials and diplomats here and could signal even more difficulties for the latest round of arms talks at Geneva, which are already stalled. Some western diplomats here said the cautious tone of the first statement may have meant that Reagan's surprise decision to dismantle the submarines was more conciliatory than had been expected and caught Moscow off guard.

Reagan's decision to dismantle the Poseidon submarine has been criticized by a number of conservative lawmakers in Washington as unwarranted because of claims of Soviet violations of SALT II.

In announcing his decision, however, Reagan said he was willing to go "the extra mile" to preserve the treaty. But he also said he would continue to abide by it only if the Soviets do the same and denounced the Soviets for treaty violations.

In a statement issued at a news briefing this evening, Lomeiko said the White House had used "far-fetched accusations" about Soviet treaty violations to confuse public opinion and camouflage a U.S. arms build-up.

Reading a government statement, he warned Washington that it cannot determine alone what is and is not a treaty violation. "One should not be deluded that the U.S. side will be allowed to determine as it thinks fit which obligations should be observed and which should not," he said.

Dismissing the decision to dismantle the Poseidon, the Soviet statement focused instead on U.S. intimations that the newly planned U.S. Midgetman missile, a small, single-warhead mobile weapon that is still in development and not expected to reach the testing stage for another few years, could be seen as a proportionate response to deployments of the Soviet SSX25, a single-warhead strategic missile that the United States claims is new and in violation of SALT II.

The Soviet statement denied the allegation, saying that the SSX25 is a modernized version of the SS13, which is allowed under SALT II.

The SALT II treaty allows each side one new missile model, aside from modernized versions of older missiles, and the Soviets claim that their new weapon is the 10-warhead SSX24. "There exists no second new type of Soviet strategic missile," the statement said. "Allusions to the Soviet missile are being made in order to stipulate a 'right' for the U.S.A. to violate one of the key provisions of the treaty."

The White House, contrary to earlier published reports, decided to proceed with the dismantling of the Poseidon missile submarine, rather than just put it in storage, in order to keep within the limits of SALT II. The deployment of a new Trident submarine later this year would have pushed the United States over the SALT II limit of 1,200 multiple-warheads, had the Poseidon not been decommissioned.

Recent commentaries in the Soviet press had been geared toward the expectation of a decision by Reagan to preserve the Poseidon and to scrap, at least partially, the provisions of the 1979 treaty. Both the United States and the Soviet Union have said they honor the treaty, even though it was never ratified by the U.S. Senate.

On Sunday, the Communist Party newspaper Pravda said the choice in Washington had come down to either dumping the SALT II provisions openly, or to shedding them, "de facto, by bits and pieces."

Earlier today, a commentary issued by Tass indicated that the Soviet propaganda line would continue unchanged, even though Washington had chosen to abide by SALT II.

The Tass article said the White House announcement yesterday was a "publicity gesture," and that its essence was "to erode and destroy everthing positive achieved in the sphere of security by joint efforts of the U.S.S.R. and the United States in the 1970s."

The article also concluded that in fact, the Poseidon will not be dismantled, but would be refitted later with cruise missiles. It also said that the same issue of exceeding SALT II limits will be faced again in 1986 with the launching of another new nuclear U.S. submarine.

The official government statement tonight refered to the Poseidon decision only in passing, and concentrated more on a history of alleged U.S. arms control violations.

In answering reporters' questions, Lomeiko said the position taken on SALT II was an attempt by Washington to "deflect attention from the negative position adopted by the U.S." at the arms control talks in Geneva.

"The U.S. at present is making a new political propaganda effort, accusing the Soviet Union of violations and presenting itself as the noble-minded advocate of the SALT agreement," Lomeiko said.