The time for change has come; the Communists must join the government," the crowd of 50,000 chanted in unison as the small man with stooped shoulders walked slowly to the front of the platform and raised his right hand in greeting.

Enrico, enrico, enrico," they yelled, as the strains of "Bandiera Rossa" and the "International" faded into the night. The sea of clenched fists and waving red flags was a rousing welcome for a weary Enrico Berlinguer, leader of Italy's powerful Communist Party.

After three days of campaigning in France and in Italy for the coming Italian and European elections, the 57 year-old Sardinian politician was running a 101-degree fever. The cool evening breeze that swept through Turin's 17th century San Carlo Square left others shivering, but under his blue wool suit and sleeveless gray V-neck sweater, Berlinguer was perspiring profusely.

With the June 3 Italian election drawing near, Berlinguer had come to this leftist-run city to plead for votes for his party.

In Turin, Berlinguer shared the platform with French Communist leader Georges Marchais. Berlinguer says he does not trust the polls that have been forecasting a Communist drop of from one to four percentage points from the unprecedented 34.4 percent that the party polled in the June 1976 elections.

Most Communist politicians , however, appear convinced that the party has lost support among the noncommunist voters who were largely responsible for its 1976 seven-point surge. So the party is concentrating on its traditional electorate, the solidly left-wing workers who make up much of the population in Turin and the surrounding highly industrialized region of Piedmont.

"Yes, I'm worried, but worried that the Christian Democrats will pick up votes," Berlinguer said. The campaign has been a rigorous one and the lines around Berlinguer's eyes are visibly deeper and his dark hair noticeably grayer than only a year or so ago.

The extent of his concern over the labor vote was demonstrated by his unprecedented lunchtime visit to the giant Fiat automobile plant.

"This is the first time ever that a party secretary has come to Fiat," said a Turin Communist official as Berlinguer's bulletproof sedan rolled to a stop, preceded by police cars and bodyguards.

At the Fiat plant, Berlinguer was surrounded by heavily armed party strongmen as he made his way to a doors. "The time for change has come, the working class must rule," some 2,000 workers chanted.

Referring to Italy's self-proclaimed "proletarian" terrrorist groups, and to charges by the ruling Christian Democrats that past Communist rhetoric was a major cause of today's violence, Berlinguer said the party's tradition was "a constructive one" and that the Italian Communists "are the country's only genuine proletarians."

After morning consultations with local party leaders, Berlinguer used the podium at both Fiat and San Carlo to lash out at the Socialists and the Radicals, two left-of-center parties that have attacked the Communists for being "undemocratic."

He saved most of his barbs, however, for the party's principal rivals, the ruling Christian Democrats.

"The giant octopus of Christian Democratic power must be destroyed," he said, "for Italy is being suffocated by its tentacles." In Turin as elsewhere in Italy, Berlinguer insisted that "Italy's problems can be resolved only if the Communists are given an equal role in a coalition with the Christian Democrats."

After their huge gains in 1976, which gave them 228 seats in the 630 member Chamber of Deputies, the Communists agreed to give essential parliamentary support to a minority Christian Democratic government. This January, faced with eroding grass-roots support, they ended this arrangement, saying they had insufficient influence on policy. Now they are insisting on Cabinet seats in the government or they will return to their pre-1976 position in the opposition.

At a dinner for French and Italian Communist workers, Berlinguer called unfounded U.S. fears that, once in power, the communists would allow Soviet influence here to expand.

"I don't see how any well-informed American can believe that," he said.

He noted that Italian Communists differed strongly with Moscow on issues such as European unity, the Camp David talks and relations with China.

"Furthermore," he added, "we are not just pretending when we say we accept our country's membership in NATO." While the party did not always agree with NATO policy, he said, it supported fully the current balance of power. CAPTION: Picture, ENRICO BERLINGUER . . . the working class must rule