The official Soviet news agency today accused President Carter of demangoguery for warning in his State of the Union speech last night that the United States would use military force to protect vital interests in the Persian Gulf region.

Moscow's initial reaction to the speech came in a Washington dispatch from the official Tass news agency, which charged Carter with "arbitratily proclaiming the Gulf area, which lies thousands of miles from American shores, a sphere of U.S. 'vital interests,'" It said the president, in a speech dealing almost entirely with the Afghanistan and Iran crisis, made "groundless assertions concerning mythical threats from any side to the movement of Middle East oil -- not to mention the U.S., of course. J. Carter alleged there exists some threat from the Soviet Union to the free movement of Middle East oil."

Tass accused the administration of assembling "the biggest ever armada of navel forces, thus actually blocking the Persian Gulf and Hormuz Strait, and it is the U.S. that is boycotting oil shipments from Iran and worked out plans for mining Persian Gulf waters."

A more authoritative Soviet response is expected to emerge in the next few days, chiefly from the Communist Party newspaper Pravda and the government paper Izvestia.

It is expected the eventual Moscow response will add up to a sharply worded rebuke, charging Carter has deliberately brought the world to the brink of a new cold war by unjustly picturing the Soviets as an expansionist military power threatening world peace. The Soviets defend their Afghanistan invasion, which has brought the Kremlin a storm of world condemnation, as a legitimate response to a call for help from the Marxist Kabul regime against U.S. and Chinese subversion.

The Soviets have not concealed their belief that Soviet-U.S,. relations, as a result of Carter's uneven foreign policy, have reached the lowest point since the cold war. The general lines of today's initial Tass response makes these points, but without the strident tone of some of the Soviet denunciations of the White House that have come in the wake of U.S. reprisals against the intervention.

"J. Carter's demagogical call for efforts to 'repel the threat' to the region can only be qualified as an attempt to distract attention from the imperialist policy of the U.S.A.

"This is also made clear by the fact that the president does not limit the sphere of U.S. vital interests to the Persian Gulf area. He spoke of Washington's determination to safeguard and defend U.S. interests not only in Europe and the Pacific but also extending through the Middle East to Southeast Asia."

Tass said the Americans are not "interested in the least how the countries which his administration intends to include in this sphere will react to this."

The agency mentioned the Soviet theme that the world of the 1980s, which the Soviet Union has helped shape with military ventures in Africa in support of what it calls "wars of national liberation," is far different from the 1950s when U.S. power was supreme.

"Washington is far from displaying an understanding of the president world and the essence of the changes taking place in it because it continues, as it did in the times of Theodore Roosevelt, to rely on gunboat diplomacy and the big policy," said Tass.