The considerable grumblings over the athletes who are not here were forgotten for at least three hours this afternoon, as Canada held one of its biggest birthday celebrations ever, by opening the 12th World University Games at Commonwealth Stadium before Prince Charles and Diana, Princess of Wales.

Games officials estimate that over the next 11 days approximately 4,600 athletes from 98 countries will compete for medals in 10 sports. The competition was to begin later tonight with the basketball events.

The biggest draw of the evening was supposed to feature the U.S. team against Benin, but the Benin team failed to show up and forfeited. Instead, the U.S. team, heavily favored to win the gold medal, played to an 88-70 exhibition victory over a Great Britain team led by Pittsburg's star guard, Clyde Vaughan.

The basketball competition is where some of the grumbling begins. It was hoped that the highlight of these games was to be a gold-medal showdown between America and the favored Soviet Union, which had a very successful tour of the United States last fall. But the Soviets, for an unexplained reason, pulled out of the World University Games last week.

The Americans seemed formidable with all-America Keith Lee from Memphis State playing pivot. But Lee did not accompany the team here, and leaves a considerable void to be dealt with by Coach Norm Stewart of Missouri.

In track and field, the 60,000-seat Commonwealth promised to be the scene of a magnificent high jump duel between China's Zhu Jian Hua, the recent world-record setter, and Canada's Milt Ottey. But Hua, like Lee, decided to skip Edmonton. Officials here hoped that U.S. sprinter and long jumper Carl Lewis would compete in the athletics events, which begin Tuesday, but he reportedly will not be here.

The U.S. women's team will perform without Mary Decker and Evelyn Ashford.

Then, there is the case of the world-famous gymnast who is here, but may not compete. Nadia Comaneci, whose greatest moments came in this country during the 1976 Olympics, has refused to say whether she will compete in the gymnastic event which begins Saturday. Comaneci has until one hour before competition begins to make her decision.

At a press conference earlier this week, she sidestepped the question repeatedly, first saying, "I am not here to compete," then saying she is "95 percent sure" she is physically and mentally prepared. Her last international competition was two years ago, at the World University Games in Bucharest, where she won five gold medals.

While Comaneci's every breath is reported here, it is her Romanian countryman Anisoara Cusmir who is busy breaking long jump records and may find her competition here a bore. "The world will be looking at me to uphold my reputation," Cusmir said.

The U.S. swimming team may also suffer because many of the top American swimmers are committed to so many different international events. Sue Walsh, Bruce Hayes and Tom Jager will compete and Mary T. Meagher may arrive at the last minute, but most of the team has little international experience.

"I'd like to pop my chest out and say we'll rip everybody apart," said U.S. Coach Sam Freas of Arkansas. "But we'll have our hands full getting medals."