To adapt an old saying, the prospect of playing host to 6,000 of the world's most influential financiers concentrates the mind wonderfully.

This week Yugoslavia will host the first annual meeting of the International Monetary Fund (IMF) and World Bank ever held in a communist country. As finance ministers and bankers from more than 130 countries begin arriving in Belgrade for preliminary meetings, an army of Yugoslav workmen are busy putting the finishing touches to the city's new airport terminal and Intercontinental hotel.

Like their counterparts elsewhere in Eastern Europe, bureaucrats in Yugoslavia have never been accused of hyperefficiency or excessive speed.

Thus the fact that it has been possible to build a luxury-class hotel from scratch in less than 12 months in evidence of the seriousness with which Yugoslav officials consider Belgrade the temporary financial capital of the world. By contrast, a project to restore a former Serbian princess' home as a museum already has taken eight years and has been held up for the last eight months because of the lack of a lightning rod.

One major reason that Yugoslav politicians are so keen to stage events such as the IMF conference is that they see international activity as a means of strengthening their country's independence from the Soviet Bloc.

The $50 million center in which the IMF meeting will be held already has housed the Helsinki review conference on European security and human rights and a conference of nonaligned foreign ministers. In addition, Yugoslavia is making a major bid for international sports events: The 1984 Winter Olympics will be held in the Bosnian city of Sarajevo, and Belgrade has announced its candidacy for the 1988 Summer Olympics.

FOR A MEDIUM-SIZED CITY like Belgrade with a population of 1.3 million, the logistics of organizing such huge events are enormous. Attendance at the present IMF conference is up about 30 perecent on the one held in Manila in 1977. All hotels within an 80-mile radius of the city have been booked fully by conference guests, which means that some bankers face a three-hour trek to and from the conference center each day.

Belgrade residents have cashed in on the occasion by renting out their homes for the duration of the conference, with a modest two-bedroom apartment going for more than $350 a night. Conference delegates are being asked to pay about $70 dollars for a buffet lunch and $30 for a single cocktail drink.

Meanwhile, a three-man official commission has been attempting to raise the tone of the city's night life. The commission is evidently under the impression that international bankers like to have dinner to the accompaniment of raucous bands and folk singing. Restaurants without musical programs have been told to organize them quickly, whatever the cost.

BELGRADE'S HISTORICAL importance stems from the fact that it occuppies a vital strategic position at the confluence of the Danube and Sava rivers, between the flat plains of Hungary and central Europe to the north and the rugged landscape of the Balkans to the south. The city has been destroyed at least 20 times during its long history, most recently in 1941 when Hitler ordered a blanket bombardment in pique at Yugoslavia's repudiation of an alliance.

The result is a capital that can hardly be called beautiful, but does have a certain charm because of the mixture of influences that have contributed to its makeup.

The rhythm of life in Belgrade is still distinctly rural, with residents rising at dawn, going to work at 6 or 7 a.m. and rushing home at 2 p.m. for a long siesta in the afternoon.

AS GOOD A GUIDE as any to the quality of life in a city is what its citizens are talking about. And the top subject of conversations here over the last few weeks has been neither the IMF conference, nor President Tito's efforts in Havana to preserve the independence of the nonaligned movement, nor even an open-air rock concert for some 70,000 teen-agers. It has concerned the nighttime exploits of a daredevil driver known as "the phantom" and his mysterious white Porsche.

To the intense embarrassment of the police, huge crowds have turned out for nights on end to watch a young man roaring about town in a stolen car and ignoring all traffic regulations. His luxurious Porsche would appear on the streets of the capital some time after 1 a.m., circle one of the city's main squares on two wheels and, following a lightning 180- degree turn in front of gesticulating police officers, speed off to a screech of tires and loud applause.

After nine successive appearances, the affair reached its climax when the exasperated police laid an ambush for the phantom before several thousand onlookers. They blocked the exit to the square with a bus into which the Porsche promptly crashed. Its driver, however, lived up to his nickname by vanishing into the crowds. The next night the expectant audience had to content itself with cheering a tractor pulling a cartload of watermelons to the morning market.