Before the future king of England could canter onto the greensward, the Rev. John Mangrum, known locally as the "polo priest," prayed over a loudspeaker to "keep the horses strong and on foot and the riders in their saddles."

Princess Diana, who has indulged her husband's passion for the ponies on dozens of occasions before, must have made a similar wish. She doesn't want a broken prince. When Charles came here to play three years ago he was hospitalized briefly with a nasty case of heat exhaustion.

Today, all was swell. The prince's team won 11-10, the horses kept their balance, and everyone stayed in the saddle.

A few television cameramen, however, were disappointed. They were hoping the royal couple's final stop in America here would produce what passes for "news" on this cushy beat.

"The prince isn't a bad guy, but I'm hoping he breaks a leg," said one man with a mini-cam. "At least let him fall off his horse. Let it be something."

In another quarter, a writer for a women's fashion magazine wondered how the princess would hit it off with Joan Collins at tonight's dinner in honor of oil magnate Armand Hammer at the Breakers Hotel. A battle of dynasties, perhaps.

Collins apparently tried calling everywhere from Buckingham Palace to the British Embassy to wangle an invitation to last weekend's dinner at the White House. She failed. In fact the royal couple heard about her various phone calls, and they were not at all happy about it. William T. Ylvisaker, president of the Palm Beach Polo and Country Club, paid the $50,000 for Collins' two benefactor tickets.

"The rumor is," the writer said, "that Joan Collins tries to upstage everyone and the princess doesn't like her one bit. She's told her friends she would really rather avoid Miss Collins."

Alas, the day went off without a hitch, athletic or social. A prop plane hired by the San Antonio-based Irish-American Unity Conference tugged a banner over the Palm Beach Polo and Country Club reading "Charles and Diana Please Help to Free Ireland," but the crowd paid it no mind. Why snap the fantasy? Especially in Palm Beach.

When the Waleses arrived at 12:30 at Palm Beach International Airport, they went through the familiar greeting drill. Dressed in ivory suitings with blue and white shirtings, the prince tried to answer the too, too impertinent questions barked at him from the crowd. Like Queen Victoria, he was not amused.

"Hey, Charles! How do you like the weather!?"

"It is getting warmer, isn't it."

"How was the trip?"

"Nice," the prince said. In truth, he seemed a bit sad.

"Do you like Palm Beach?"

"I've been here before."

What a sparkler. Well, it has been three weeks on the road.

The princess mixed it up with rather more spirit, accepting a bouquet of wilting flora from little Tara Sansbury, the daughter of a county administrator. In a pearl-gray dress of unutterable indescribable magnificence, the princess played her part.

As she passed the carnations back to her lady-in-waiting, Diana was assaulted with more plant life from the eager crowd. A local kindergarten class presented her with a half-dozen bouquets and a copy of "The Velveteen Rabbit."

"All these presents for me?" the princess said. "It's like Christmas." And then she passed the loot back to her attendant.

At the airport a trio of bagpipers in McPherson tartans played "Balmoral" as the royal couple worked the crowd in their individual ways. Jim McVeigh, a specialist at making his pipes sound like a gang of gagging tabbies, thought the princess was just grand:

"She's a fantastic gal, isn't she? She asked me, 'Isn't it a little hot to be wearing your plaids?' What a woman. The prince? To tell you the truth, I don't know where the hell he disappeared to."

Before the polo game began in the afternoon, the Miami Dolphins cheerleaders gyrated, Mickey and Minnie Mouse perspired, and an all-black high school band played to the all-white crowd in the grandstand. Florida Gov. Bob Graham and selected socialites rode by the infield in truncated late-model Cadillacs. Helen Boehm, of porcelain fame, flashed a ring the size of a baby's fist and reflected a rainbow.

The crowd of about 10,000 was the biggest to see a polo game in decades in this country. Many institutions of higher learning, including West Point and Cornell, once played polo, but the game began to die out with the disappearance of cavalry warfare. Officials here are hoping against hope that this game, with all its attendant hoopla, will make polo as accessible to the masses as baseball.

Among the spectators at the match were Eva ("I'm not Zsa Zsa") Gabor, Gregory Peck, Collins and Merv Griffin -- all guests at the Breakers bash. The rest of the crowd, dressed in every shade of pastel, roared at the prince's every move, however slight.

The prince played the No. 4 position, a defensive role similar to that of a safety in football or a fullback in soccer. Though his polo game is best known for boring the princess to distraction, he is said to be more than competent at the sport. He is rated as a 4 out of 10, better than 85 percent of all other players.

The program described the prince's sporting ability this way: "HRH is coming off a successful season. At Windsor, England The Prince of Wales captured the Silver Jubilee Cup for England over the team representing Brazil . . . Those who have the honor of playing alongside HRH recognize him for his courage, love of the game, courteous manner and exemplary horsemanship."

The players, four on each team, played six seven-minute "chukkers," changing horses after each. Although the horses are called "ponies," they are nothing of the sort. They are huge, muscular beasts intent on sprinting and slamming their way into the hearts of princes and lesser men.

The prince played with Geoffrey Kent, Ylvisaker, a one-time star player for the 69th Regiment Armory in New York, and Memo Gracida, one of the highest ranked players in the world. Gracida fairly carried his team, and the prince, to be charitable to one who needs no charity at all, was mainly ornamental.

The opposing team comprised four all-stars, according to those who know about such things. They seemed rather polite to royalty, making sure the prince took no fliers from his horse. His teammates paid him their constant respects. Whenever any of them shouted to him, they cried, "Look out behind you, sir!" or "Here it comes, sir!"

The prince was concerned about his play. He did not bring any of his own ponies to Florida for the match, and so he had his polo manager, Maj. Ronald Ferguson, inspect the horseflesh here.

But it had a happy ending. And as the sun began to set the princess presented the winning team with a porcelain and silver trophy. The prince was even luckier. He got a trophy and a kiss.

The princess and Collins showed up at tonight's dinner in outfits that expressed their public personalities almost perfectly. Collins, appearing with her spanking new husband, Peter Holm, stepped from her limousine in a black gown with a neckline that plunged sharply toward the southern hemispheres. A gaggle of diamonds glittered at her throat.

Then came the lighter side of life. Preceded by a motorcade of a half-dozen motorcycles and a "dummy" stretch limousine, Diana emerged from her Rolls-Royce wearing a cerise panne gown with a neckline fit for a schoolmarm. The dress was backless, but somehow a study in innocence.

The prince wore black.

The dinner benefited the Armand Hammer United World College of the American West in Las Vegas, N.M.

Bob Hope, one of the evening's entertainers, said, "Diana would make a great film star. She reminds me of Madeleine Carroll from before the War."

Preceding the royal arrival was a royal fracas in which the likes of Mary Sanford, the "Queen of Palm Beach," decided not to attend the festivities. One Palm Beach socialite, Mara Ann Goodman, seemed to apologize for the disturbing events:

"I hope the prince and princess have enjoyed our humble little town."

Dozens of Palm Beachers lined the streets to catch a glimpse of the royal couple, and at the Breakers, the lobby was jammed with guests and a few Instamatic-wielding Anglophiles.

Seconds after the princess sashayed by, guest Madelaine Allison looked as though she were ready to faint. "I feel like history walked past me. She's a perfect Cinderella."

Some of the guests seemed to take the evening pretty lightly. Some arrived in normal Dodges and Oldsmobiles, and some of the dresses seemed hauled from the backs of society closets.

Guests at the dinner included Peck, Gabor, Abigail Van Buren (Dear Abby), Marvin Davis, Sen. Paula Hawkins, Ted Turner, Sen. Sam Nunn and Sir Oliver Wright, the British ambassador. Cary Grant had been expected, but didn't show because of illness.

While the town of Palm Beach is as remote from Middle America as Bonwit's is from K mart, the evening's entertainment was pure cornpone: Hope told jokes, Victor Borge tickled the piano, and Merv Griffin, well, did what he does.

Of the nearly 400 people who attended the dinner, only 32 couples paid the benefactor's share of $50,000 a couple. The rest paid $10,000 a couple.

After the dinner, Hammer thanked the prince and the prince thanked Hammer. Charles' toast was rendered in a tone rather more impassioned than usual, as he sprinkled his speech with liberal "for God's sakes" and even one indiscreet "hell."

In answer to those who have criticized United World Colleges as elitist, the prince said, "Elitism! You hear this all the time, that it's an elitist college! . . . What I want to know is what's wrong with an elite, for God's sakes?

"How on earth does anyone expect to get anything done in life without an attempt to educate people's characters and minds? After all, there's so much to be done in this world."

Of the workload at the six branches of UWC, Charles, a Cambridge University graduate, said, "My God, you have to have ability to handle the very, very tough course . . . I know I couldn't have done it."

And with that, the multicourse dinner ended.