The man leading Sicily's fight against the Mafia

''It's hot in Palermo,'' the chief prosecutor's chauffeur says. He isn't talking about the weather. The Fiat he is driving has no outward sign it is an armored car. But when the driver rolls down the window, he points to the inch-thick glass. ''Bullet-proof, '' he says.

His boss, Vincenzo Pajno, chief prosecutor of Palermo since 1980, is a very busy - and well-protected - man these days. He is responsible for prosecuting 366 leaders of the Mafia indicted by Italian authorities recently. (Some 200 of these people were already in jail.) Mr. Pajno and his 12 deputies are at the cutting edge of Italy's effort to counter a threat of national importance.

His base is the ''Palazzo di Giustizia'' (Palace of Justice) here in Palermo, a white marble building of the ''Italian fascist'' school of architecture that was much in vogue in the 1930s - huge, square, and forbidding on the outside, cavernous and bleak on the inside. Two young carabinieri with machine guns slung over their shoulders usher a visitor in.

Prosecutor Pajno is a small, well-groomed, efficient-looking man in a gray suit. He is polite and serious. But he is beginning to see some progress in his ''caccia alla Mafia'' (Mafia-hunt); by the end of the interview, he has begun to smile.

He starts by drawing parallels between the gangsters of the United States and the Sicilian Mafia. He explains that in 1980, several laboratories used for the manufacture of heroin were discovered and shut down in or near Palermo. That was when the serious Mafia hunt began.

Another big change came in September 1982 with the murder of Gen. Carlo Alberto Dalla Chiesa and his wife. Up to that moment, all of the ''vittime illustri'' (illustrious victims) slain by the Mafia had been Sicilians. Here in Sicily they were highly respected men, and their loss was deeply felt. But all of Italy felt the blow when General Dalla Chiesa was killed.

Dalla Chiesa, who had been named police prefect of Palermo in the fight against the Mafia, was considered a national hero because of his outstanding work against the terrorist Red Brigades during the 1970s. A native of the northern Italian province of Piemonte, he had had a distinguished career in the carabinieri - a branch of the Italian Army that functions as a police force and that is deeply respected on the national level.

With Dalla Chiesa's death, public opinion in all of Italy was mobilized against the Mafia as never before.

Today, Chief Prosecutor Pajno, though obviously not a man given to overblown expressions of enthusiasm, chooses to describe the results of his and his colleagues' all-out effort against the Mafia as ''extremely positive.''

The total number of homicides within the Palermo jurisdiction has decreased notably. From one of the files on his desk, Pajno produces figures to bear this out: From January through October of this year there were just 58 murders in this area, compared with 101 in 1981, with 152 in 1982, and 108 in 1983.

Meanwhile, many of the 366 suspects being sought or awaiting trial are considered to be very important members of the Mafia. About 60 of these have been arrested as a result of the ''singing'' of Mafia boss Tommaso Buscetta, who was extradicted from Brazil earlier this past summer.

Mr. Buscetta's evidence also has enabled the investigating judges to verify and corroborate many facts and details concerning Mafia crimes of which there had heretofore been insufficient proof.

Although these suspects are scattered in a number of prisons throughout Italy , their prosecution and trial are being treated by the judiciary as one case, since there are so many links among Mafiosi and their networks of criminal activities.

Pajno's role is similar to that of a US district attorney. He evaluates the evidence prepared by investigating magistrates and decides whether charges should be pressed and a suspect brought to trial.

He cannot afford too much delay. A law that went into effect last August could limit the period of time during which a suspect can be held for trial after he has been accused by the prosecutor.

A number of suspected Mafiosi have been in custody for many months, and the fear has been voiced that they could be released before they are brought to trial. However, the prosecutor also has the authority to decide in each particular case whether this possibility of release should be waived.

''The Mafia is a very complicated problem,'' Pajno says. ''It is as much a problem of the United States and the world as it is of Sicily, because the problem of narcotics is international. Ultimately, it has to be stopped at its source: in China, in Thailand, in Peru - wherever the raw materials come from.''

Pajno speaks of collaboration among the US Drug Enforcement Agency, the Federal Bureau of Investigation, and various branches of the Italian police. He is particularly pleased that the US and Italy now can extradite members of the Mafia from each other's countries for questioning.

''The recent extradition treaty between Italy and the US,'' he says, ''opens up new possibilities in our investigations and allows for greater speed in our work.''

When his visitor gets up to leave, Pajno's sense of courtly solicitude and chivalry, so much a part of the Sicilian character, comes to the fore. He instructs his driver, who has been sitting in the anteroom, to take her back to her hotel in his car.

The car is among those parked at the top of the neat white steps that lead up to the Palazzo di Giustizia's entrance. Concrete ramps have replaced the marble steps at both sides of the palazzo's facade, so cars can pick up their passengers at the very doors, then glide away as if descending the steps.

These are the cars of the magistrates, and the security of their owners is a top priority. Twelve high-ranking magistrates and police officers have been shot and killed in Palermo since 1970 - many of them in their cars - including two of Pajno's predecessors, Prosecutor Pietro Scaglione in 1971 and Prosecutor Gaetano Costa in 1980.

It is getting dark as we glide down the ''steps'' in the magistrate's armored car. The Palazzo di Giustizia sits right at the edge of a teeming, noisy, and squalid part of town.

Across the street, laundry festoons the walls of run-down tenements. The trees along the streets are so thick that the sidewalks are dark before it is dusk. Heady odors of freshly baked bread waft from the local panificio (bakery), and fish are sold from wooden stands right in the street. This is the Palermo of the Palermitani.

Traffic is heavy, since all the shops are open following the afternoon siesta. We inch down a side street and circle the huge, round, 19th-century theater in Piazza Giuseppe Verdi.

''Whenever I drive for the magistrate,'' the chauffeur explains, ''we have an escort of four motorcycles in front and four of them behind. We keep the siren going, and we never stop for anything. It's dangerous driving that way, of course. But then, these are very tense times in Palermo.''

Back at the hotel, the car door feels very, very heavy as it slams shut.