The Soviet Union tonight attacked yesterday's joint declaration on arms control by the seven participants in the Williamsburg summit as nothing more than "camouflage for the unbridled and dangerous arms race."

A statement issued by the government news agency Tass labeled the declaration an endorsement of the Reagan administration's plan to deploy new medium-range nuclear weapons in Western Europe.

Arvid Pelshe, at 84 the oldest member of the ruling Politburo, died Sunday. Obituary on Page B5

The seven major industrial nations made it clear that they "will continue to expand nuclear arsenals and carry out extensive programs of deploying the latest types of arms," Tass said.

The declaration issued yesterday at Williamsburg, reportedly after considerable debate, called on the Soviet Union to "contribute constructively" to negotiations to remove nuclear missiles from Europe. But it added that if the United States and the Soviet Union fail to reach agreement on limiting the missiles, "the countries concerned will proceed with the planned deployment."

The meaning of the Williamsburg declaration is "obvious," the statement said, "by using words about peace the seven want to camouflage the unbridled and dangerous arms race."

The quick Soviet rejoinder appeared to have been designed to counter the impact of the Williamsburg declaration on arms control on western public opinion. It was preceded by a series of Soviet commentaries suggesting that the summit had failed to resolve economic issues dividing the participants and that the arms control declaration was adopted under strong U.S. pressure.

Tass said the summit document's stated readiness "to cooperate with the Soviet Union in arms reduction" could be "only welcomed if the words of the western leaders accorded with their deeds, if they really were referring to the finding of just accords" observing the principle of "equality and equal security."

But, Tass continued, President Reagan's resolve to deploy new MX missiles and the joint pronouncement on the deployment of new U.S. missiles in Western Europe "are at variance" with the pronouncements on arms reductions.

It reiterated Moscow's earlier charges that the United States has been pursuing "an obstructionist line" at the Soviet-American arms talks in Geneva and "that is the reason why the talks became deadlocked."

The "essence" of the Williamsburg declaration, Tass said, is the endorsement of the Reagan administration's plan to deploy Pershing II and cruise missiles in five West European countries and shift the military and strategic balance in favor of the West.

"That is an absurdity but it is contained in the statement itself," Tass said. "It is openly said in it that the relevant negotiations on this question in Geneva 'will determine the level of deployment' of these arms and that if agreement on this score is not reached then the NATO countries will proceed with the planned deployment of the U.S. systems in Europe at the end of 1983."

On the eve of the Williamsburg summit, the Soviet government raised the possibility of introducing nuclear weapons in Eastern Europe in an effort to counter "an additional threat" to Soviet security interests posed by the Pershing II and cruise missiles.

Tonight's statement did not repeat this Soviet warning. The tone of the statement was relatively moderate and appeared to be designed to refute western arguments rather than frighten western audiences.