As the deadline for participation in the Moscow Olympics arrived today, the Soviet Union declared that American efforts to organize a massive boycott of the Games have "completely failed."

Brushing aside the fact that more than 50 nations will not attend, including three of the top five winners at the 1976 Games, the Soviets asserted that those joining the boycott are isolated malcontents wishing only to wreck international sports.

The official Tass press agency said "athletes have categorically rejected" the Carter administration's "blackmail and threats." Japan's decision to join the boycott has received no comment from the Soviets so far.

Although most Islamic nations, virtually all Asian nations, as well as many African and South American countries have decided not to attend, Tass said, "Washington is irritated by the fact that one after another, countries of Western Europe, Asia, Africa, Latin America, give rebuff to U.S. diktat."

In Moscow's eyes, the Carter administration has suffered a significant defeat because most of the West Europeans have refused to boycott. Nonparticipation by Canada, West Germany and Japan is criticized but generally ignored or painted as the actions of Carter's lackeys.

The presence here this summer of teams from France, Britain, Australia, Italy and other important capitalist nations cannot but dissipate Washington's intended heavy blow to Soviet self-esteem, in the view of observers here.

In dealing with the Soviet attitudes, the leadership here has as its disposal massive propaganda and a well-cultivated sense of psychological isolation from the rest of the world. The official media have played on mass notions of embattlement in its domestic accounts of the boycott, painting President Carter as the villain.

"If he hadn't used our Afghanistan assistance as an excuse, he would have some other reason," a Muscovite said today in a casual street conversation when asked about the Japanese boycott decision.

But finding a culprit in best Soviet style has done little to relieve the general feelings of disappointment here that the Americans are not coming.

Part of the reason may be that while they do not necessarily connect the boycott to the invasion, the Soviets are upset at every level about the Afghan intervention. It is difficult to find anyone here who is openly pleased or boastful about the situation. To a degree, this can be attributed to the isolation created for the Soviets by their own media. But Afghanistan is remote from averge Soviet perceptions, not automatically thought of as a fraternal socialist state, and known to be backward and needing massive aid from Moscow.

The battlefield deaths in Afghanistan, at a steady if unknown rate, make everyone here uneasy. Some senior journalists in recent conversations dismissed as Western fabrications reports of many hundreds of deaths, and admonished a Western correspondent from even asking on the ground that such information here is "a military secret." But elsewhere, several other official sources conceded uncomfortably that the toll from Afghanistan is at least "in the dozens."

Other sources say party lecturers in factories and institutes have made unconvincing presentations about the intervention. One Moscow factory worker said recently he was infuriated by the explanations.

"It is not like 1968, when we knew and supported the action in Czechoslovakia," he said. "That was a socialist state we had nurtured, and it showed itself ungrateful. But who knows anything about Afghanistan?"