On the eve of tomorrow's long-awaited opening of the Olympic Games, Moscow's hotels, restaurants and stores are spilling over with fresh meat and fish, imported delicacies and special canned goods seldom seen here in such profusion.

But less than 20 miles from the city center, where the Olympic bunting, slogans, flags and police begin to thin out, fresh meat and vegetables are not to be found in the state's chronically undersupplied food stores. Despite the impression of relative plenty the capital exudes, there are many mid-summer signs pointing to mounting shortages of better food across the nation in the months ahead.

Soviet agriculture in 1980 is struggling without promise of great gain against the combined effects of a long, cold spring, an unsettled summer that has brought damaging drought, widespread flooding and frequent hailstorms, and the American feed grain embargo in retaliation for the invasion of Afghanistan.

"The consequence of a hard winter can still be felt," Economic Gazette summed up this week in a key article disclosing substantial decreases in meat and dairy production for the first half of the year. Meat production last month alone was about 11 percent below the June 1979 level, in part a result of the distress slaughters in carefully nurtured livestock herds early this year after the United States held up delivery of more than 10 million tons of extra cattle fodder.

Although foreign guests at Moscow's best hotels can find virtually every variety of traditional Russian dairy products available at their tables, in the nearby suburban city of Muitishi, only three kinds of milk or sour cream could be found on the shelves of a large state store this morning. Soviet citizens from other large cities report rationing of milk is widespread, and freely available only to young children.

The Economic Gazette reported that June dairy production was about 8 percent lower than of last year. Overall through 1980, Soviet milk production is about 4 percent lower. Western agricultural sources say the new figures indicate the continuing effects of a poor fodder harvest last year, the long, wet spring that kept stock from forage, and the scarcity of feed gain aggravated by the U.S. embargo.

"There is seldom fresh meat here in our city," said a woman in Muitishi, "but we are used to it. Same for vegetables." The meat counter was empty, although the fish counter was piled with two varieties of fresh fish and stacks of sardines in oil stood on canned food shelves nearby.

Unofficial sources say severe shortages of fresh food have been the cause of wildcat strikes this spring at the Togliatti and Gorki auto plants. Unconfirmed reports of similar work stoppages elsewhere in central Russia have continued to trickle into Moscow in recent weeks.

The Soviet Union keeps state reserves on hand to handle such emergencies, and sources say these were used to quell the workers' discontent in the spring. But the issue of better food supplies now is near the top of the summer's internal concerns of the leadership. The Communist Party's paper Pravda and the government paper Izvestia this week urged agricultural workers to redouble their efforts at ensuring a good harvest. The Soviet Union reaped 179 million tons of grain last year, a disaster, but this year, Western agricultural sources estimate the total may reach as high as 225 million tons if conditions improve.

However, Izvestia, quoting central statistical board figures up to mid-July, placed the harvest so far at just over 8 million hectares, a little more than half the area harvested at this time last year. A rainy summer has left large areas outside the black earth zone of European Russia flooded. The paper said good wheat and barley crops in the Don River region had been knocked down by high winds and a Ukrainian paper this week mentioned serious problems from hail damage.