MUSEO DEL PRADO - A Threatened Treasure

Never have so many paintings been surrounded by so many people in such murky circumstances. The Museo del Prado stands as unquestionably one of the world's greatest art museums; but it also stands in a thicket of problems that get in the way of presenting its best art to the public.

No one is more aware of the problems heaped upon the 164-year-old Prado than its director, Alfonso E. Perez Sanchez. And he and the Prado are taking steps to correct most of them.

A round-faced man who combines a harried sense of pressure with a sweet patience, Mr. Perez Sanchez answers questions about the overcrowding, poor lighting, and other environmental predicaments at the Prado without a murmur of dissent. He knows the museum is taxed to the straining point. But he hopes the current efforts to reconstruct and relight the museum will help.

''Twenty-five years ago, Madrid's atmosphere was limpid and pure,'' he observes during a hurried interview in his office, bedecked with some treasures from the museum's collection. ''But the city has become industrialized, cars have brought pollution . . . and this has affected the paintings.

''It is a grave situation,'' he notes. ''You can see an opacity and a stiffness in many paintings.'' Many of them, he says, may have been damaged beyond repair.

Pollution is only one of the problems the museum is trying to combat with its nearly $21 million overhaul. Along with the installation of a new climate control system, the project also includes new lighting, security systems, and inner-wall construction. The job, which Perez Sanchez calls ''a work in progress,'' is about two-fifths completed. But since much of the work so far has been with unseen systems and wall reinforcements, it's hard for visitors to see much progress.

The result: A walk through the Prado today is a mixed blessing. The unpainted ceilings and splotchy, burlap-looking wall coverings in the unrenovated wings provide a depressing ambiance for the paintings and sculpture. The old lighting, too, plays havoc with many paintings in the museum, casting a glare on some and leaving others in a shadowy funk.

The difference between the old and the new lighting - as in the gorgeous Tintoretto collection, where the Italian master's colors use the light brilliantly - is, in the words of the director, ''like night and day.''

To art lovers from all over the world, such matters are clearly outweighed by the presence of all the Rubenses, the Goyas, the Velazquezes, and the tortured, luminous El Grecos that leave you fumbling for words to describe them. But it is difficult - nearly impossible, in fact - to get around one of the Prado's knottiest problems: the mob scenes.

Whatever day of the week you choose to come, at whatever time of day, you will find yourself constantly bumping into people, especially in the most popular rooms, those devoted to El Greco, Velazquez, and Goya. Just laying eyes on the paintings can frequently be quite challenging. The gift shop, too, is swamped with tourists frantically fingering the oil-and-canvas reproductions.

This situation is nowhere more apparent than in the separate Villanueva Building, around the corner, which houses Picasso's legendary and politically explosive ''Guernica.''

The exhibit built around this painting - which, after spending 42 years in New York's Museum of Modern Art awaiting the end of Spain's right-wing dictatorship, was returned to Spain in 1981 in accordance with the painter's will - includes sketches and paintings that led up to the final, monumental masterpiece. In the side exhibits, you can walk through the workshop of Picasso's mind and see him hammer out questions of composition and form that were to affect his work for decades to come. You can see the painting coming together, piece by brilliant piece, in his eye.

Seeing the painting itself, however, is another matter.

Placed behind a protective housing that extends some 20 feet into the room, ''Guernica'' draws a file of onlookers every day, frequently two or three deep, who stand or pass in front of it. You can stand with them and look at the whole painting. But you cannot get back far enough to take in the whole sweep and power of the thing, in the way it should really be seen.

''Oh, yes you can,'' Prado director Perez Sanchez retorts, somewhat facetiously. ''If you stand in one particular corner long enough, the crowd will part briefly and you will see it all.''

In discussing the range of quandaries the museum is wrestling with - ''problems facing every great musuem in the world . . . the Louvre, the Vatican'' - he acknowledges that overcrowding is one of the most stubborn.

''Right now, we have a crowd-control system that will not allow more than 2, 000 people in the building at one time.'' But what he does not have, and desperately wants, is the ability to control crowds in key rooms.

The museum is now open from 10 a.m. to 7 p.m. Tuesday through Saturday and shorter hours on Sunday. It's closed on Mondays. Merely ''increasing the hours will only increase the number of visitors. That has been the experience.''

The matter currently rests somewhere in the Ministry of Culture - because it involves hiring enough guards to staff the doors to these rooms. The director has no idea when it will be resolved: The Prado has not been master of its own purse strings since 1968, when the Franco government handed much administrative and artistic control over to a National Board of Museums.

But the issue goes beyond making it easier to appreciate the art. Overcrowding threatens the paintings themselves, because of excessive carbon dioxide from museumgoers' breath. And that makes the business of protecting the paintings more difficult.

For the time being, though, Perez Sanchez and others have to content themselves that the murky old Prado is at least slowly donning a new suit of lights - and taking a breath of fresh air.