Orson Welles, 70, an artistic genius who at the age of 25 wrote, directed and starred in the 1941 American film classic "Citizen Kane," was found dead by his chauffeur yesterday in a bedroom at his home in Los Angeles. He had been suffering from diabetes and a heart ailment, and the Los Angeles coroner's office said he appeared to have died of natural causes.

Mr. Welles also broadcast the famous 1938 Halloween radio adaptation of H.G. Wells' "The War of the Worlds," which panicked listeners on the East Coast who did not hear his preliminary announcement that the program was fictional and took literally his news account that aliens from another planet had landed in New Jersey.

He is generally considered to have been one of the giants of the American film industry, but all his best work was done before he reached the age of 30, and the last 25 years of his life were characterized by a series of aborted or unfinished projects, television appearances that failed to impress critics and commercial endorsements of such products as wine and sparkling water.

"I started at the top and have been working my way down ever since," Mr. Welles often said, only partly in jest.

Nevertheless, he was honored in 1975 with the American Film Institute's annual Life Achievement Award "for superlative artistry and versatility in the creation of motion pictures."

His film, "Citizen Kane," based on the life of newspaper publisher William Randolph Hearst, is considered by many to have been one of the great classics of the American film industry -- a 1962 poll of international film critics called it "the best film in motion picture history." It brought Mr. Welles an Academy award for the script and a nomination for his acting.

Although he had never appeared before a camera, Mr. Welles was given carte blanche by the studio, RKO, to select the subject, treatment and cast, and the result was a film that traced a newspaper magnate's rise to power as his idealism becomes corrupted by materialism. Dying alone, the old man's last word is "Rosebud," and a journalist assigned to discover its meaning reconstructs Kane's life through a series of interviews with people who knew him.

Eventually he abandons the search, and the movie ends with a casual destruction of many of Kane's possessions, including a sled he had used as a boy. As the sled is being tossed into a fire, the camera catches a word painted on its side, "Rosebud," just before the letters are dissolved in flames.

The film included several technical innovations that have influenced film making, including the use of wide-angle and deep-focus photography and bizarre sets.

But it did not impress Hearst, who tried and failed to prevent the film from being released and then banned both the mention of the movie and Mr. Welles' name from all his newspapers.

Born May 6, 1915, in Kenosha, Wis., George Orson Welles was a child prodigy who by the time he was 11 had been around the world twice with his father, a manufacturer and an inventor. He was acting on the stage in Dublin when he was 16 and two years later won critical acclaim in the United States for his portrayal of Mercutio in "Romeo and Juliet."

Critics said he had a "splendid purple-velvet voice," that made him an immediate success on the radio, and his voice soon became known to millions as that of Lamont Cranston on the radio mystery series "The Shadow."

With John Houseman he codirected a brilliantly staged production of "Macbeth," for the Federal Theatre Project's Negro People's Theatre, presented in the spring of 1936 at the old Lafayette Theatre in Harlem.

The next year Mr. Welles and Houseman launched the Mercury Theatre which produced a stunningly successful modern dress adaptation of "Julius Caesar," that more closely resembled conditions in Fascist Italy than ancient Rome, and the following year the Mercury company presented the famous "War of the Worlds" broadcast.

To give the story a sense of immediacy, Mr. Welles decided to tell it as if an invasion from Mars was actually under way in the United States. News flashes were written in, accompanied by "calming" messages from government officials, and the result was a nationwide panic in which thousands fled their homes, prayed in the streets or volunteered for military service.

It also got Mr. Welles on the cover of Time magazine -- at the age of 23 -- and it led to the RKO job in Hollywood after the Mercury Theatre project collapsed in financial distress.

But that was the pinnacle of his career, and critics and biographers have argued and debated since over the reasons for his decline.

Mr. Welles' own version was that "I had too much power . . . . I had luck as no one had. I had the worst bad luck in the history of the cinema, but that is in the order of things. I had to pay for having had the best luck in the history of the cinema."

After "Citizen Kane," two subsequent films, "The Magnificent Ambersons," and "Journey into Fear," were badly edited and relegated to double-billing status. He made a low budget "Macbeth," then starred with his second wife, Rita Hayworth, in "The Lady from Shanghai."

He produced a screen version of "Othello" in 1952 that was made largely with his own money earned from smaller parts in several other movies, but the film did share the Grand Prix at the Cannes film festival of that year.

His last American film was "Touch of Evil" in 1957, but he directed several European films during the 1950s and 1960s, including "The Trial" and "Chimes at Midnight."

In the later years of his life, Mr. Welles had gained substantial weight -- almost to 300 pounds, and he developed a fondness for long black cigars. He was married three times, to actress Virginia Nicholson, then to Hayworth and finally to Italian countess and actress Paola Mori. He had a son, Christopher, by his first marriage; a daughter, Rebecca, by his second marriage; and a daughter, Beatrice, by his third marriage.