THREE YEARS AGO a writer visiting Rudolf Serkin's famed summer festival at Marlboro, Bt. reported, "Everyone here is talking about the fierce talent of pianist Cecile Licad: It is her second year here, and she is still only 17 and she wears braces. She comes from the Philippines but has lived with her mother in Philadelphia for the past six years."

Then they thought of her as still a child. Those days are over. There are no braces. Perhaps she is not a classic beauty, but she was striking indeed as she strode intently on stage in the Kennedy Center concert hall Monday to begin her first rehearsal ever there -- with the National Symphony under Mstislav Rostropovich for four concerts, including one today.

She addressed the piano without a word to any of the players practicing around her, sat on the stool and for 15 minutes worked at some of the many strenuous passages of the work she is performing, the Saint-Sae ns Concerto No. 2. She wore an expensive-looking dark wool dress and high-heeled brown leather boots. Her pitch-black hair hung to her waist. She lay her black leather purse under the bass register of the keyboard. She seemed oblivious to everything but the keyboard. Finally she replied "hello" to Rostropovich as he came out to begin.

Those cloistered days at Marlboro are over. Her manner reflects both the intensity of her concentration and the intensity of the strain, and in particular the loneliness one may experience in an abrupt transition from the classroom to the major concert hall.

This year she became the first artist in 10 years to win the prestigious grand prize of the Leventritt Foundation. Over the years it has been a lauching pad for, among others, Istomin, Weissenberg, Cliburn, Graffman, Perlman and Zukerman.

If the Leventritt doesn't carry quite the publicity value of the Tchaikovsky competition, for instance, it offers more in the way of career guarantees. There is the automatic set of concerts with the New York Philharmonic. For Licad, these came about a month ago, to rave notices. Now it's the National Symphony (again to a rave), followed by the Chicago with Solti, the London Symphony with Abbado, and so on. There is an exclusive contract with CBS Masterworks, management by Columbia Artists and a $10,000 prize.

In an interview after that first rehearsal in Washington she described, in sometimes halting English, what it is like. "Suddenly life is very different. I guess I get nervous. In two weeks it's another concerto. In three weeks it's another concerto, and so everything in my head is going around and so I don't know what to expect. It's so much pressure and extremes."

The pressure was, if anything, worse last Tuesday, when just before her first public performance at the Kennedy Center, four angry demonstrators rushed down an aisle screaming in protest at the presence in the presidential box of Imelda Marcos, wife of Philippines President Ferdinand Marcos. Early on in her career the Marcoses took a special interest in Licad, who was named "First Piano Scholar" of the Philippines Young Artists Foundation.

Making the evening even harder was the fact that the Saint-Sae ns concerto is one of the first large works that Licad (pronounced lee-KOD) has "studied myself," without the guidance of her celebrated mentor, Rudolf Serkin.

Licad was a triumph Tuesday night. There were ovations both from the audience and from the orchestra musicians. When this kind of talent surfaces, the question is not so much whether it can develop as how to shape it best.

Right now Licad is having to work hard just learning to cope with the personal problems of touring. For instance, she was being escorted the short distance from the auditorium to her room at the Watergate simply because there had not been time for her to learn the route on her own. And that Monday afternoon she was temporarily at a loss to find a piano where she could practice uninterrupted for several hours.

She recalled a similar moment of panic on another tour several years ago. "I believe it was one night in Michigan. It was so foggy that no one could meet me at the airport where we came in. The city was about an hour away or something. It was my first time to find a place for myself in a hotel. They hadn't reserved a room for me. And it was hard for me."

Licad came to the United States at 12, with her mother, and auditioned for Serkin at the Curtis Institute in Philadelphia, which he then headed. She took lessons with two of the best teachers around, Mieszyslaw Horszowski and Seymour Lipkin. Serkin regularly checked on her progress. Three years ago he accepted her as a full-time student and she moved to Vermont to be near his home.

Serkin has gone to great lengths to see that Licad's way of playing follows the direction of her own gifts and does not imitate any others, including himself. None of the three concertos considered for here and New York -- the No. 2s of Chopin, Saint-Sae ns and Rachmaninoff -- is in his active repertory. And she does not now perform those monoliths so closely identified with Serkin, such as Beethoven's "Emperor" and the two Brahms concertos. As Licad observes, "That's the thing. I shouldn't be like him. He makes me be my own self."

Until this year her concert experience has been mostly in chamber music (twice here at the Smithsonian with the Music from Marlboro group) and in solo recitals at less populous cities on the national circuit.

The latter may have helped prepare her for at least some of the pressures that come with touring alone, as she is doing here. "There was a time in Hawaii. I forget the name. I told them that I wanted to try the piano before I play, so they said to me that the piano tuner would be there at 9 and would be through by 10. So I came at 10:30 and the piano tuner was there, but he was not through until 12. Well, in any case, I said that I must try the piano if it's tuned. And I said keep the people out for a while because I am practicing. Anyway, they finally opened the doors, and there were only two people waiting at that time. And there was the piano tuner as well."

Her victory in the Leventritt grew out of many of these events, because that is where the performers are judged, over substantial periods of time and in a variety of circumstances. Unlike the other major prizes, the Leventritt dispenses with the grueling ordeals -- and, some would argue, the test-tube artificiality -- of the formal auditions squeezed into a brief time frame. Licad doubts that she would do as well in those contests and plans to enter none. She says that Serkin has told her she does not need to.

But there is not a touch of arrogance in the way she says this. At this point she is so tense about assuming this new public role that it seems that this is pressure enough. She does not yet have that flair for showmanship with which a Rubinstein, a Horowitz or a Cliburn can make the life of a performer seem easy.

When does she play best? "Just to enjoy it," she answers, "without thinking of being judged."