President Carter yesterday told a group of American athletes and their elected representatives that the United States definitely will not participate in the Summer Olympics in Moscow.

Speaking emphatically and sometimes somberly to approximately 100 current and former Olympic athletes and coaches in the East Room of the White House, Carter appealed for support of a boycott of the Games to punish the Soviet Union for its invasion of Afghanistan.

However, he made it clear that, regardless of the athletes' attitudes, the United States would not participate in the Moscow Games.

An informal poll taken later by the athletes and coaches indicated that 44 opposed a boycott and 29 supported the administration's position, while 24 abstained.

At the White House, Carter told the athletes and coaches: "I can't say at this moment what other nations will not go to the Summer Olympics in Moscow. Ours will not go. I say that without any equivocation. The decision has been made.

"The American people are convinced that we should not go to the Summer Olympics. Congress has voted overwhelmingly, almost unanimously, which is a very rare thing, that we will not go. And I can tell you that many of our major cities, particularly those democratic countries who believe in freedom, will not go."

Judging from the audience's reaction to Carter's 20-minute address, the decisiveness of his message was not lost on them. Those who had harbored hopes that Carter might change his mind if world tensions ease before the May 24 deadline for Moscow entries realized that they have been grasping at a nonexistent straw.

"He closed the door," said Willie Davenport, a gold medalist in the hurdles in the 1968 Olympics and a member of the U.S. four-man bobsled team at last month's Winter Olympics at Lake Placid, N.Y. "Not only did he close the door, he locked it and threw away the key."

Jane Frederick, the American record-holder in the pentathlon, said later at a news conference called by the athletes: "He was telling us definitely, 'This is what you're going to do. It's okay if you speak out against it, but it won't have any effect.'"

The athletes and coaches were briefed for nearly two hours yesterday -- first by national security affairs adviser Zbigniew Brzezinski on the Soviet invasion, and then by White House deputy counsel Joseph Onek on the Olympic boycott and the administration's plans for alternative international games in late August.

When Carter entered the East Room for his speech, the only part of the session open to reporters, he was greeted with a stony silence that veteran White House observers say is rare, if not unprecedented.

At the conclusion of his remarks, he received polite applause. Reporters then were asked to leave while the president remained briefly to answer questions.

Asked politely why U.S. amateur athletes had been "neglected" in the past by the government, the president took issue with the word "neglect," according to those at the session, but he said, "I don't think we have done enough."

Asked what he wanted the athletes to do, the president, repeating a theme of his speech, reportedly said: "You have your independence to do what you want, and we hope you agree with us."

Carter reportedly said he lacks the legal authority to order the athletes not to go to Moscow, but that he does have "the authority, by declaring a national emergency, to stop all travel to the Soviet Union."

He said he did not think such drastic action would be necessary because he expects the United States Olympic Committee to abide by his decision.

The USOC must formally decide within the next two months whether to accept an invitation to the Moscow Games. The organization's leaders have told the president that it will comply with his request.

The USOC's 482-member House of Delegates will consider the matter April 11-13 in Colorado Springs. The organization's administrative committee recommended last weekend that a decision be deferred as long as possible in the hope that the situation in Afghanistan changes, causing a shift in American public opinion and the president's support of a boycott.

Carter's forceful, resolute speech yesterday could prompt a change in that recommendation, however, paving the way for the House of Delegates to decline the Moscow invitation, USOC souces said.

USOC Executive Director F. Don Miller, who has said he would try to "keep all options open," was unavailable for comment. He is in Brussels for discussions today with representatives of about 15 national Olympic committees.

Spokesmen for the USOC's 47-member Athletes Advisory Council, which attended yesterday's White House meeting and is to meet today in Rosslyn, said that group has not yet formulated an official position on a boycott.

In his speech, the president told the athletes:

"There must be a firm, clear voice of caution given to the Soviet Union, not just an admonition and criticism of what they have already done to despoil a small and relatively weak country, but to make sure they don't look upon this as an achievement without serious adverse consequences which can then be followed up with additional aggression along the same lines."

The president said he was not "naive," and that he realized that the alternative games the adminstration is attempting to organize would be no substitute to Olympic competition. But he urged the athletes to regard the boycott positively as a means of "having helped to preserve freedom and having helped to enhance the quality of the principles of the Olympics, and having helped in a personal way to carry out the principles and ideals of our nation, and having made a sacrifice in doing it."

"The president was very impressive. He changed some minds," said Terry Anderson, a coach of the U.S. shooting team. But the athletes' poll suggested that, to many of them, the boycott is a symbolic action, rather than a substantive one.

Still, yesterday's message to the athletes -- that the president will not change his mind about the Olympic boycott, no matter what happens in Afghanistan -- came across loud and clear.

"It was kind of like an ultimatum, or final decree," said modern pentathlete John Fitzgerald. "I think every athlete was hoping that maybe we could go, maybe something would happen to change things. But it was plain and simple -- no matter what happens now, we're not going."