Pope John Paul II made a strong pledge in his first extended speech as supreme pontiff yesterday to continue the liberalizing work of his immediate predecessors.

He called for "the most exact execution" of the decisions of the Vatican II Council that moved in the 1960s to open up the Roman Catholic Church to the modern world.

Although he said this must be done in a "prudent" way, John Paul II immediately took a stand against the group of cardinals who had clearly indicated that they saw the election of a new pope as an opportunity to undo the work of Vatican II.

"It is necessary to place oneself in harmony with the Council," the new pope said to the cardinals gathered in the Sistine Chapel, where they had elected him the previous evening. "One must put into practice what was stated."

Cardinal Giuseppe Siri of Genoa, leader of the ultraconservatives in the College of Cardinia, indicated his displeasure with John Paul's speech. He was extremely curt with reporters who asked for his reaction to the out-come of the conclave. He answered questions almost monosyllabically.

Asked if he had liked the pope's speech, he said. "I can't remember now" and cut off any further questioning.

His behavior was in sharp contrast to the expansive mood of nine American cardinals who gave a press conference to express their joy and satisfaction over the elections of Cardinal Karol Wojtyle of Krakow, Poland, as pope.

A French cardinal, Jean Guyot of Toulouse, hinted broadly to French reporters that the Polish cardinal had in fact been proposed by the Americans as a way out of the impasse after all the leading Italian candidates had been burned out in the first day of voting on Sunday.

But the cardinals were still bound by pledge of secrecy on the proceedings of the conclave. Cardinal John Carberry of St. Louis said, "I would just love to be free to tell you every detail, and it would thrill you. But I'm too scrupulous."

Most of the American cardinals said that Wojtyle had visited their dioceses and that they had gotten to know him well on his frequent tours of American Catholic centers in recent years.

Cardinal John Dearden of Detroit said: "We thought we couldn't reach the same heights of enthusiasm as last time, but we have. Most of us know him very well." They spoke repeatedly of the new pope's "common touch."

It seems clear that even if the Americans were not actually the ones to put Wojtyl's name forward, they have enough collective clout in the church to have been able to block his elections.

Cardinal John Krol of Philadelphis, the only Polish-American cardinal, said that the fervor of the Catholic Church under pressure was a factor in the choice of Wojtyla, "but not a controlling factor." Echoing an insistent chorus by cardinals of all nationalist Krol insisted that international politics did not enter into the decision. "We chose a pope for the church," he said.

Krol's parents come from the Krols region, and he has visited Wojtyla in his diocese.

Various cardinals also rejected suggestions, contained mostly in questions by American reporters, that the choice of a Pole to head the church could place it on a collision course with the Communist government of Eastern Europe.

Most European reporters seemed to take the opposite tack. II Messagers, Rome's largest daily newspaper, headlined an editiorial favorable to Wojtyle election. "Factor or Detente."

East European affairs analysts here said they were sure that Wojtyla's election would be a great stimulus to the church in regions like Slovakia in Ezechoslovakia and the Ukraine in the Soviet Union as well as in Poland, but that they doubted that the church wanted a confrontation with Communist authorities.

Cardinal George Flahiff of Winniper said he hoped that Communist governments would even regard Wojtyle choice as "a positive gesture" that could lead to better relations. Clahiff contrasted Wojtyla to the primate of Poland. Cardinal Stefan Wojtynski noting that the new pope had never soug

Flahiff said the Wojtyla was on lists of possible popes even during the August conclave, but that no one restyl took his name seriously because they all assumed that the pope would be another Italian.

The one point on which the Cardinais seem to be making no secrets is the idea that the Italian cardinals killed each other off. On Sunday night, said the French Cardinal Guyot, "We were all balled up, and we didn't even know whether we were going to stay in Italy." The feuding among the Italians apparently made the choice of a non-Italian the only way out.

Most Italian political analysts agree that the effect of electing a nonItalian as pope will be to remove the Vatican's preponderant influence in the country. One of the most frequently heard comments is that the ruling Christain Democratic Party has been orphaned by the loss of the pope as its father image.

"The Vatican," editorialized the left-of-center newspaper Republica, "has for the past thirty years, for better or for worse, been a point of reference for the largest Italian political party. Today, that point of reference no longer exists. This finally obligets it to act for itself as a responsible adult."

Catholic leftists here seemed almost university enthusiastic about the new pope, but the leadership of the Communist Party was adopting a wait and see attitude. The initial thinking around Communist Party headquarters, as expressed privately by party leaders, was that the pope's new imability to intervence directly in Italian affairs will be good for the party.

As far as East West relations are concerned, the Italian Communists expressed confidence that the new pope would continue to be, as he was in Poland, a negotiator and a man of dialogue with Communist authorities.

The analysts assume that the Polish government, despite its congratulatory telegram, is worried because whatever strengthens Polish nationalists since the church has been so highly identified throughout Polish history with resistance to outsiders. Polish ntionalism traditionally worries the Russians, and the Polish Communist government has made it its basic policy rule to stay on good terms with Moscow.

Britain's Cardinal George Hume, the archbishop of Westminster, speculated to reporters that the new pope would want to continue Paul VI's East-West policies. "The Poles, more than any other nation," said Hume, "have learned to live with ideologies that aren't theirs. I would think he is a man who would want to make things work."

Hume underlined the passage is John Paul II's speech yesterday in which he said, "We have no intention of political interference, nor of participation in the working out of temporal affairs."

But there were other ways of reading the speech. John Paul II also spoke of extending his hand to help all those "who are oppressed by whatever injutices or discrimination whether it has to do with economy, life in society, political life or the freedom of conscience and just religious freedom."

"We can be very sure that he will be very firm on human rights," said Detroit's Cardinal Dearden, who conceded that the help the pope was offering would be available to the peoples of Eastern Europe.