The Soviet Union has begun to signal its belief that the death of Egypt's Anwar Sadat is a fresh opportunity to reassert Moscow's influence in that key Middle Eastern country and that it expects, at the very least, a gradual shift in Cairo leading to a more anti-American Islamic world.

After a period of relative restraint, the Soviet press during the past two days has blossomed with commentaries and reports about the situation in Egypt that blame the Reagan administration for "crude interference" in its internal affairs and denounce "sinister" plans for an alleged U.S. invasion of Libya.

The official Tass news agency charged tonight that the United States was "feverishly stepping up war preparations in the Middle East" and described planned arms deliveries to Egypt and Sudan as "extremely dangerous and reckless playing with fire."

These pronouncements lace together two central themes that have emerged from Moscow's reaction to the assassination of Sadat.

First is that the United States has lost not only its key ally in the Arab world but also the political leader who helped reconcile Washington's determination to continue unqualified support for Israel with the requirement to maintain a viable U.S. political position in the Middle East.

The second is that the new Egyptian leadership -- despite its professed wish to continue Sadat's pro-Western orientation -- will want to distance itself from the United States and move close to the Arab world.

While there is no evidence that a major policy reassessment is under way in Moscow, the press reports seem to reflect an effort to keep everybody in the region off balance. Moreover, the Soviets appear to feel that both the recent events and time play into their hands.

As a result the Soviets have cautiously reasserted their intentions to reenter the Middle East dispute with a government-to-government note to the United States that asserted that "what is happening around Egypt cannot but affect the interests of the Soviet Union's security." However, they have refrained from bellicose statements of intentions to counter U.S. moves.

Instead, one of the main thrusts of Soviet propaganda in recent days focused on Arab pressure on the new Cairo leadership to return to the Arab family. This included kid glove treatment of President-designate Hosni Mubarak and the presentation of Sadat's assassination as the work of his political opponents rather than of an isolated religious fanatic.

Another theme centers on what is seen here as an unstable political situation in Egypt. American military moves are perceived as an effort to shore up the new leadership, "to ensure its adherence to the Sadat's course," as Tass put it.

Privately, the Soviets say that in the highly personalized politics of the Arab world few men could quickly acquire Sadat's charisma and boldness and pursue policies that have isolated Egypt from even such moderate states as Jordan and Saudi Arabia.

Neither the United States nor Moscow knows for sure what Mubarak's policies will be. While Egypt's new leader received much of his military training in the Soviet Union and was a frequent visitor there, he was in the forefront last month of an Egyptian move to expel all Soviet technicians and reduce by half the Soviet Embassy staff.

According to diplomats, Moscow expects the departure of Sadat to terminate the Camp David process. It also sees the conflict over the sale of AWACS planes to Saudi Arabia, other complex issues such as the form of future Palestinian self-determination and the status of Jerusalem, and the hard-line West Bank policies of the Begin government as combining to complicate Washington's position in the region.

Moscow has already renewed a call for an international conference on the Middle East.

Seen from here, according to political observers, Sadat's departure has not produced any efforts to define a new U.S. policy toward the region. Rather, the Reagan administration appears to be focusing on military measures to shore up U.S. positions.

This is derided by Soviet commentators. Tass for instance, said that "it would be well for certain persons to remember that neither the deliveries of the latest weapons nor the permanent presence of tens of thousands of U.S. military advisers could save the shah from the anger of the people and help the United States preserve its domination over" Iran.

The military moves taken so far, the commentator said, "cannot prevent the growth of the same internal dissatisfaction that led to the fall of the shah."