CHARIOTS OF FIRE," the superb new British film about two Olympic sprinters, starts off with a rhapsodic slow-motion sequence of a pack of barefoot runners gliding across a beach, bodies taut and straining with ecstatic pain, mud splattering their white uniforms. The stillness and quietude of that motion is reinforced by an elegiac melody that curls out from under the runners' feet, incredibly simple synthesizer lines converging in a suddenly heroic theme.

It's an electric texture that at first seems oddly modern for a film so painstakingly accurate in its '20s setting, but the neo-classical mood and moment are triumphant: One is swept along with the runners and it's easy to find oneself suddenly breathless. Vangelis Papathanassiou, the Greek composer and keyboard artist who created the soundtrack for "Chariots of Fire" (Polydor PD1-6335), remembers the first time he encountered the unscored scene. "It was really beautiful and I had this idea for the theme . If you analyze it, it's really innocent, childish, simple, like a children's song. The most difficult thing is a simple thing."

Vangelis (which is how he's known, for obvious reasons) has created one of the most alluring soundtracks in recent years, an aural reinforcement of "Chariots" visual eloquence, capturing the sense of competitive urgency and triumph in a less gauche manner than Bill Conti's theme for "Rocky." Like its overblown predecessor, "Chariots of Fire" has hit a common nerve with the public: It's already been a Top Five album in England and is getting increasing air play in America as the film opens in more cities. And like the Pachebel "Canon" used in "Ordinary People," the "Chariots" theme is sliding into popular usage as a background to newscasts and sports programs.

The 38-year-old composer, who left a politically turbulent Greece in 1968, is not a newcomer to soundtracks, having started out with Frederic Rossif's "Apocalypse des Animaux" in 1972; more recently his music was featured on PBS's "Cosmos" series. He's actually better known as a pop artist (solo albums and others cut as leader of Aphrodite's Child have sold more than 20 million copies in Europe) but Vangelis' sterling evocations on "Chariots" may attract more work than he wants: already he's scoring Costa-Gavras' new film, "Missing," as well as Ridley ("Alien") Scott's future-thriller, "Blade Runner."

Speaking from his London recording studio, Vangelis described his methodology on "Chariots," which occupied him for almost two months. "When I work, I stop being intellectual, I just be physical. I use my intellect afterward, to analyze what I've done. The first time watching the partially edited film I had a reaction, and I put it down immediately on tape. It seems somehow that the first take is right. Since I play all the instruments myself, the tape is like a score.

"I try to put myself in the situation and feel it . . . I'm a runner at the time, or in the stadium, or alone in the dressing room . . . and then I compose . . . and the moment is fruitful and honest, I think." There are moments of remarkable tension in the film, particularly in dressing room scenes where the loneliness of the runners is underscored by an ethereal electric piano line that excludes all other noises.

"The loneliness of the runner Abrahams is incredible," Vangelis points out. "Every runner is lonely, they are very special people. But I don't want to describe tension, or describe anything. This is the mistake most people make. When you describe something, it kills it. If tension becomes a cliche', as it has in horror films, then you're not doing a good job."

"If you are compatible with a director then everything seems easy. We had the same sense of how to achieve certain things. The dressing room, to me, is a very tense situation. The more simple you make it, the more people can get involved in it. You suggest a few things, and then the audience does the rest. If you say everything, then you don't leave any room for feeling. To have a great economy may be the best way to have the best participation."

Several years ago, Vangelis explained on an original album titled "China" that Chinese music was "an image of the universe whose object was not to please the senses but to convey eternal truths, music to order the inner spirit through it's moral effect." The loneliness of the runners in "Chariots" was matched by the loneliness of the composer in his "laboratory," but like the runners in the film, Vangelis emerged triumphant, cradling a gently heroic theme that has its desired uplifiting moral effect.