When the Moscow Philharmonic and conductor Gennady Rozhdestvensky performed earlier this year in Belgrade's 4,000-seat Sava Center, they were heard by an audience of only 700. Last week, in the course of a monthlong European tour that will end with its return to Washington tomorrow, the National Symphony Orchestra and its music director, Mstislav Rostropovich, visited the Yugoslav capital and filled the same hall to capacity. One night later in Ljubljana, with the ovation of another sold-out house ringing in his ears, an ebullient Rostropovich admitted that the difference between 4,000 and 700 was music to his ears.

"This will show you what a . . . personality I have," he chortled, "but of all the things that we have done on the tour, I am proudest of this. Here is important Soviet orchestra and best Soviet conductor, and they play for only 700 people!"

Upstaging Soviet artists while politely thumbing his nose at what he now describes as his "former country" are nothing new for Rostropovich, but the opportunity to do so at close range had been denied him until now. The concerts in Belgrade and Ljubljana marked the first time that Rostropovich has performed in a communist country since his departure from the Soviet Union 11 years ago.

Not that the point of going to Yugoslavia was to embarrass any particular country or to make unflattering comparisons between ideologies, as Rostropovich took considerable pains to emphasize following the Ljubljana concert in a toast he made at a dinner hosted by American Ambassador John Scanlan. Far more important, the conductor stressed, was the recognition that, regardless of our political systems, we have a common humanity and many shared cultural values that can be expressed through art.

Still, it was impossible to overlook that, for the concerts in Yugoslavia, Rostropovich chose to program symphonies by the two greatest Soviet composers of the century -- Dmitri Shostakovich and Sergei Prokofiev -- both of whom were officially censured by the Soviet government in 1948 for "decadent tendencies" in their music. It was just as hard to miss the sarcasm in Rostropovich's allusion to that event.

"Both dedicated their works to me -- Shostakovich, his two cello concertos, and Prokofiev the "Sinfonia Concertante" -- and both were my dearest friends. But Stalin and Zhdanov decided they could teach music to these two great geniuses of our century." After a moment, he added, "I am happy to be in America now, because President Reagan doesn't try to teach music to me. I teach music to him."

Used to being the center of attention, Rostropovich is among the most widely traveled classical musicians in history. One might have expected his reaction to Yugoslavia to be fairly cool; in fact, there was a childlike exuberance in his manner that belied his years of touring experience. Part of the reason was his unconcealed joy at being once again in a Slavic country; part was clearly a response to the warmth of the Serbian and Slovenian audiences that had come to hear the orchestra's performances.

Yet even in his high spirits, Rostropovich kept a measure of ironic detachment. Strolling through the center of Ljubljana after the concert there, accompanied by Virginia Mars, the orchestra's president; Stephen Klein, it's new executive director; and a few friends, and humming to himself whenever there was a gap in the conversation, Rostropovich caught sight of an austere, hulking monument that dominated a park near the hall. "That's socialist realism," he said drily. "I feel like I'm back in my own country."

Apart from such reminders, Yugoslavia was a welcoming stop on the tour. Spotlessly clean, its shops full, its people dressed fashionably (many in jeans), Ljubljana exuded a charm and vitality that immediately put one at ease. Rush-hour traffic breezed through the city without delay, and the only lines were at stops of the modern and efficient public transit system.

That meant that for the members of the orchestra -- half of whom were staying at the top-of-the-line Ljubljana Holiday Inn -- the Slovenian capital was pretty much like any other place on the tour -- a place to go, check into a room, play a concert, try to find something to eat, try to sleep a little, wake up and leave. The National Symphony has acquired a reputation, not exactly envied by other American orchestras, of saddling itself with back-breaking tour schedules and this one -- which included stops in London, Paris, Berlin and at festivals in Montreux and Lucerne, Switzerland; Athens; Stresa, Italy; and Belgium -- was no exception.

Indeed, on the day the orchestra left Washington, several members of the Pittsburgh Symphony, playing a concert in Montreux halfway through their own European tour, took pity on their NSO colleagues for having to spend four days in Sicily and four in Athens, and play 23 concerts in 29 days.

But play them the orchestra did, and not all that badly, according to Klein, who made no secret of his satisfaction at being associated with an orchestra that could play as well as the NSO has on tour, despite the difficulties its schedule presented. While there were the usual complaints about hotels and difficulties with organization, some members of the orchestra seemed utterly unfazed, taking advantage of the long stays and idle hours to go sightseeing or work on their tans. Nor did the heavy travel burden deter others from doing even more traveling: Contrabassoonist Lewis Lipnick, a BMW owner, rented a BMW at one stop on the tour and took associate conductor Andrew Litton for a spin on the Autobahn at 180 kilometers an hour, a mere 120 mph . . . Later, he came across a little town outside Ljubljana called Lipnic. "Finally, I know where I come from," he recounted, clearly delighted at having discovered Yugoslav roots.

Litton, exploring on his own, found himself in a town where the only telephones were in the town's one hotel when he remembered he had to file a report with WMAL. Undaunted, he bribed the concierge for the use of a room with a phone, promising to have a meal in the hotel's restaurant if he were allowed to make the call.

Listeners to WGMS had their reports delivered under somewhat less harried circumstances by Program Director Paul Taire, a veteran of many NSO tours and an important friend of the orchestra.

Other reports will doubtless be making their way to Washington via the State Department and the United States Information Agency, for as always when the orchestra tours, it fulfills a diplomatic as well as cultural mission. In Yugoslavia, that point was noted by Scanlan, who called the members of the orchestra "our best diplomats," as well as by a spokesman for the Slovenian government, who praised the orchestra for its musical excellence and acknowledged the value of its concerts as a cultural gesture from the United States.