The Kennedy Center celebrated 10 years of art and tumult on Saturday night, its Roof Terrace dotted with Washington's glamor class. Its leader of a decade must have been moved.

Not so. "I've got a bad leg," said Kennedy Center chairman Roger Stevens, the lights of Rosslyn shimmering in the haze beyond him, "and I feel like hell."

The celebration, after the opening performance of Leonard Bernstein's "Mass," may not be remembered as this social season's grand kickoff. Some thought it would be. But by 11 p.m., there were 12 empty tables and a noticeable lack of Reagan administration stars who, fleeting or not, make or break parties. And where were the Kennedys?

Two distressed members of the social order discussed the turn of events.

No. 1: "Jazzy this is not."

No. 2: "Where is the administration?"

No. 1: "I mean, look, let's face it. This should be the beginning of the season. Sparkling, exciting. If this is the way it's going to be, I say let's go back to our summer retreats. If you use my name, I'll kill you."

A pause for reflection.

No. 1: "What do you think it is?"

No. 2: "I don't know. I can't figure it out."

No. 1: "Maybe it's the tennis match. Makes you not want to come out again."

No. 2: (Sighing.) "Reentry is difficult."

But then came the arrival of Leonard Bernstein, superstar. Things perked up. You could congratulate him and find yourself in a headlock grip. Or you could watch the wiggling crowd cling to him, vaguely reminiscent of pet guppies around fish food.

"Where's Lennie?" one guest inquired.

"Oh, he's probably hugging someone somewhere," responded New York producer Richmond Crinkley.

He was. Arriving like a monarch returned to his homeland, Bernstein received the masses. Everyone in his line of traffic was hugged and kissed, once on each cheek, male and female alike. He talked, too. Most everyone got a tidbit.

"Are you going to come to Moscow?" asked Donna Hartman, wife of Arthur, the American ambassador to Paris.

"Not if I can help it," he replied. "But if you're there . . . "

"Is this your greatest work?" a radio reporter asked.

"Oh, come on," said Bernstein. "Who's counting?"

At one stop in the crowd, a local photographer handed Bernstein a snapshot of Bernstein's daughter, Jamie, singing at the Kennedy Center honors ceremony last year. Her father is crying in the background.

"I just took it because I wanted to capture the moment," said the photograher, James Howard.

"Put the picture in your pocket, Dad," instructed Jamie, who performed in "Mass."

"I'm glad you liked it, sir," said Howard.

Then Bernstein walked away. "That picture meant more to him than words," Howard said. "I just wanted to capture that love between a father and daughter."

The party was outdoors, where a breeze blew long dresses and mussed precision hairdos. Several hundred guests negotiated crab claws and snails in a fattening butter sauce. Among the guests were several senators, Joan Mondale, arts philanthropist David Lloyd Kreeger, presidential secretary Helene Von Damm and White House deputy press secretary Larry Speakes.

One wondered if the administration notables had paid special attention to some of the "Mass" lyrics that seemed particularly directed at them:

"All you people of power,

your hour is now,

"You may plan to rule forever,

but you never do somehow."

"The country's not in an economic mess," Speakes said, responding to a question asking if it were. "It's on the way out of an economic mess. We're right on target. Just get these budget cuts and the message to Wall Street . . . "

Feeling quite well about the economic situation was Rep. Charles Wilson (D-Tex.), one of the few Democrats who voted for Reagan's budget cuts but against his tax cut. How does he feel these days?

"Very wise," he said.

At Wilson's elbow was his date, Lady Coates. She said she was once married to British nobility.

"Going out with Charlie Wilson is like going out with Texas," she explained. "He's big."

And how did Wilson find his lady?

"Just a good eye," said Wilson.

"How did you find me?" asked Lady Coates.

"Matter of fact, you found me, darling," said Wilson.

Much of the party conversation was equally intellectual, ranging from life on the Vineyard to where Walter Mondale was. "He's just not here," said his wife, who later added, to no one's suprise, that her husband is "considering" running for president. "I think he would be a wonderful candidate," she said.

The other de rigeur topic was the show itself. Some liked it, but others remarked that it was too long, too "heavy," or too outdated.