Vladimir Horowitz, the eccentric virtuoso of the piano whose extraordinary personality and skill overwhelmed six decades of concert audiences, died suddenly early yesterday afternoon at his home in Manhattan, apparently of a heart attack.Though standard biographies list his birth date as Oct. Horowitz recently celebrated what he called his 86th birthday. Horowitz virtually cornered the market on celebrity among 20th-century pianists.It never did go over that line, but there was the sense of an unbelievable energy being harnessed, and the felling that if he ever let it go, it would burn up the hall." The pianist Andre Watts said yesterday that Mr.Horowitz "was like a demon barely under control out there on stage." Indeed, Mr.His presence hovered over several generations of pianists who followed him.

The secret lies mainly in contrasts." The Horowitz style was not admired by all.

Horowitz's playing of such standards as Tchaikovsky's First Concerto and the Rachmaninoff Third electrified listeners.

He was also famous for his high-powered versions of Mussorgsky's "Pictures at an Exhibition," Sousa's "Stars and Stripes Forever" and the Liszt Sonata. Schonberg, long the senior music critic for The New York Times and the author of "The Great Pianists," wrote: "As a technician Horowitz was one of the most honest in the history of modern pianism.

Even his frequent retirements from performing had a romantic appeal to mass audiences. Horowitz quit playing in public four times--between 19, from 1953 to 1965, from 1968 to 1974 and from 1983 to 1985. His return to Moscow and Leningrad in 1986, after a 61-year absence, became a major media event reported around the world.

Born Into a Musical Family He was named Vladimir Gorowitz when he was born in Kiev, in the Ukraine, into a prosperous and cultured family. His mother and sister, Regina, were pianists, a brother, Georg, a violinist. Horowitz altered the first letter of the family name for his Berlin debut in 1926. In the political upheaval, his family members lost most of their possessions, and Mr.