Sir Oswald Mosley, 84, a British aristocrat who embraced fascism in the 1930s and thereby discredited what once had promised to be a brilliant career in his nation's politics, died in his sleep yesterday at his home at Orsay near Paris.The cause of death was not reported.

Sir Oswald, who was born into a wealthy family in Staffordshire, England, and who inherited a baronetcy dating from the 18th century, was said to be the only man considered a likely candidate to be British prime minister in either a Conservative or a Labor government. That was in the 1920s.

In the 1930s, disillusioned with the inability of the conventional parties to deal with the crisis of The Great Depression, he struck out on his own. An admirer of Benito Mussolini -- and later of Hitler -- he formed The New Party, a neo-Fascist group, and later the British Union of Fascists. For these activities he was imprisoned for several years during World War II. He had lived in self-imposed exile in France for most of the time since then. His following was minuscule and his name anathema to many of his countrymen.

In the 1930s, Sir Oswald imitated German and Italian models of fascism, dressing his followers in black uniforms and adopting the stiffarmed Fascist salute. Handsome and athletic despite a World War I injury that left him with a permanent limp, he was regarded as one of the most effective orators of the day.

He used these gifts not only to act like the European dictators, but to advocate many of the policies they also favored. He was an anti-Semite, blaming Jews for the things in the world that did not suit him. His asserted that international Jewish financial interests were trying to promote war with Nazi Germany.At the same time, he claimed that Jews were behind "the international Communist conspiracy."

In 1936, he led his followers on a march into heavily Jewish neighborhoods in the East End of London. The result was violence. After that incident, uniforms were outlawed for British political parties and remain so to this day.

In an autobiography published in 1972, Sir Oswald sought to justify his Fascist career. Many reviewers found him unconvincing and one described it as "a grotesque, ghostly giggle from an appalling past." Writing in The Washington Post, Kenneth Rose said, "There is a formidable weight of evidence to disprove Mosley's defense, and I for one remain unconvinced of the fundamental benevolence of British fascism."

But Rose also said of Sir Oswald: "No man ever squandered a glittering armory of gifts so wantonly, or on so worthless a cause." On hearing of Sir Oswald's death, Lord Shinwell, an elder statesman of the British Labor Party, said he would have made "a great leader of the Labor Party if he had kept his head and not got fanciful ideas."

If Sir Oswald placed himself on the outer fringes of British politics, he remained close to some of the most eminent figures of the social establishment throughout his life. His first wife was Lady Cynthia Curzon, a daughter of the Marquess of Curzon, and their marriage was attended by King George V and Queen Mary of Britain and by the Belgian royal family. His second wife was Diana Mitford, one of the famous Mitford sisters. He was a friend of the Duke of Windsor, who shared his administration for things German.

Sir Oswald was educated at Winchester School and the Royal Military College at Sandhurst. He was commissioned into a fashionable cavalry regiment, as befitted a man of his station, and transferred to the Royal Flying Corps in World War I.

He was first elected to Parliament six weeks after the war ended, at the age of 22, and his skill in debate and ability in other matters were widely remarked by the elders of the Conservative Party. Two years later, he became an independent and then a member of the Labor Party.

In 1929, he received a cabinet post. He fashioned a program to deal with the worsening economic situation, advocating heavy government borrowing and heavy expenditures on public works. By today's standards -- or even by the standards of the New Deal -- it hardly seems radical. But it was rejected by the leaders of the government. Arrogant and impatient, Sir Oswald left the Labor Party and embarked on his career as a Fascist.

Sir Oswald had four sons, two by his first marriage and two by his second.