"Sandro, Sandro," the crowd chanted rhythmically, bursting into applause as the wizened man with glasses appeared in the doorway of the plane arriving in Milan for a ceremony. Whenever he goes, Italy's 83-year-old Socialist president, Sando Pertini, can count on such a reception.

The widespread admiration here for Petini contrasts with Italians' general contempt for politicians.

Some political leaders have been booed while appearing with Pertini and many are irritated by his privileged position in his countrymen's hearts. Several have accused him of overstepping the narrow constitutional bounds of the presidency in this parliamentary system.

But affection is strong for this former resistance leader whose political life has never been touched by scandal.

This popularity is confirmed by public opinion polls and by the size and enthusiasm of the crowds that turn out to see him. Close to 70,000 Italians, singly and in groups, have received audiences with the president since his July 1978 landslide election by the Italian parliament.

Some analysts believe Pertini's popularity reflects disillusionment with a political stystem that has given Italy governments with an average lifespan of 10 months. It also appears to indicate that despite frequently voiced cynicism, large numbers of Italians are still capable of political hero worhsip.

"Italians need something and someone to believe in," said the energetic president during a recent 45-minute chat.

Dressed in a three-piece gray plaid suit and holding an ever-present pipe, he spoke with warmth about Italian youth, today his overiding passion. Pertini has received 27,000 students at the Quirinale Palace in less than two years.

Clearly pleased by his popularity among this age group, Pertini appears to consider it almost a mission to combat disaffection by giving youngsters faith in their country and political system.

Pertini appears determined to act as a moralizing force.'During the latest government crisis, which came on the heels of several national scandals, he repeatedly urged the prime minister-designate -- with what he terms partial success -- to choose as ministerss only men with unblemished careers.

But the major concern of this grandfather figure clearly are the children. He said his biweekly exchanges with groups of youngsters sometimes give him cause for alarm. On this occasion he was troubled by an 11-year-old boy who told him his father was a crook and that he hated him, by a 15-year-old girl who said she was pessimistic about life and by two teen-age girls sympathetic to the jailed terrorists.

Answering the youngster's questions from Italian scandals to international politics, he is particularly eager to denounce terrorism in a country where many people are enraged by continuing social injustices.

Pertini exhibits a deep faith in both mankind and the democratic process and deep grief that so much of his time has been spent attending the funerals of terrorist victims.

"There are only two really decent men in Italy today, the pope, who is Polish, and Petini," said a Roman carpenter not long ago.

"Pertini is like a flower in a sea of mud," said a retired Milanese factory worker.

Comments of this sort, have been repeated frequently during the last 18 months. Some point to his political independence that has often put him at odds with his own party, as during the 1978 kidnapping and eventual killing of Aldo Moro when Pertini opposed the Socialists' call for negotiations with the Red Brigades.

Pertini also won points from the emotional Italians when he moved into a hospital in April 1979 to be at the bedside of a dying friend, veteran Republican leader Ugo la Malfa. But the high point of his popularity came last fall when he used his personal prestige and position as head of the armed forces to resolve a long-smoldering conflict over demilitarization with Italy's air controllers that had threatened to halt all air traffic. The move by Pertini contrasted sharply with years of government inaction.

"Finally, someone who has clear ideas and the courage to act on them," an employe of the national airline, Alitalia, said at the time.

Pertini, however, does have critics. The other day a Rome traffic police officer who served in the Italian Army from 1940 to 1944 censured the president for his constant referrences to the resistence.

"It wasn't my fault Italy then had a fascist government," he said. "Thousands of us risked our lives doing our duty, but the president acts as if we didn't exist."

But most of the opposition to Pertini comes from other politicians. Some think he is too old for a job that includes such key powers as appointing a prime minister-designate and calling new elections when necessary.

Politicians have accused him of excessive emotion and impulsiveness -- he has made public statements about uncompleted judicial processes, for instance.

As the first president elected with the support of all major parties, Pertini is credited by all with devotion to the constitution. But observers like Communist Sen. Emanuele Mascaluso believe Pertini's activism has become particularly noticable in a situation of political instability. The presidency has become an unprecedented point of reference.