Mancham, the swinging president of the recently independent Seychelles Islands, was overthrown early today in a coup he insists was Soviet-inspired.

In London for a conference of Commonwealth leaders, Mancham was having breakfast at the Savoy Hotel, where he is staying in a $250 a night suite, when he was interrupted by a British Foreign Office official with the news that he had been ousted.

"It is a Judas story," said the 37-year-old international playboy, poet and politician - the latest of a number of heads of state to be toppled while traveling abroad and the second to suffer this fate while away for a Commonwealth meeting.

He was ousted by supporters of his prime minister, Albert Rene, leader of the Marxist Seychelles Peoples Union Party which was in a coalition with Mancham's Democratic Party.

Rene, 41, assumed the presidency. He said in a broadcast over Seychelles Radio that "the people have overthrown Mancham" and he had agreed to form a government.

Indicating a swing to the left, he promised policies that would make the islands "free of capitalists and foreign countries" and said he would issue more detailed statements within 24 hours.

Seychelles Radio also said without giving any reason, that several British officials were detained and would be deported. About 20 people were reported to be involved.

[The Associated Press, quoting sources in Victoria, capital of the Seychelles, said that two persons were killed in an initial attack on a police barracks.]

Mancham said "Mr. Rene and his group were dining and eating with me" two days before he left for London. "They came and kissed me goodbye at the airport."

Jimmy, as he is known in dozens of nightclubs around the world, is famous for much more kissable partners than Marxist politicians. He has been linked in print with a Yugoslav actress, a New York jewelry designer and Fiona Richmond, a star of the undressed stage here.

Mancham had been unelected president of the Seychelles, a strategically situated Indian Ocean archipelago of 58,000 inhabitants 1,000 miles off the East African coast, since their independence from Britain last June 28.

The islands, famous for their spice, were first colonized by the French in 1768 and were taken over by Britain in 1794.

Mancham, the son of a Chinese immigrant, was chief minister of the islands before independence and as such became president when the island became an independent republic within the Commonwealth. The first post-independence elections had been scheduled for 1979.

An unsigned statement broadcast over Seychelles Radio this morning attributed Mancham's downfall in part to "a style of life which involved lavish spending when the country and the people are working hard and making sacrifices."

Mancham, according to the broadcast, never spent three consecutive weeks at home.

But Mancham, talking to reporters in the cramped offices of the Seychelles High Commission, here said:

"My life style is the life style of everybody engaged in the leadership of the Seychelles. I've had that reputation [of playboy] since I left school. It has been my life style all my life . . . (Rene and company) live in a more lavish style. They drive bigger cars."

Mancham, who had driven from the Savoy in a gray Rolls Royce and was dressed in a tailored blue suit and modist blue shirt with a white collar, explained that, of course, he was on the road a lot. He was, he said, also foreign minister, and "we can't live in isolation. I did a good job."

Details of what happened in Victoria are still obscure. But from what Mancham got from the Foreign Office and from Sepchelles Radio - overseas telephones were cut all day - some 200 followers of Rene's party marched on the police station, where the island's weapons are stored.

The station was seized and then, in turn, the radio and airport. That was enough for control since the Seychelles have no armed forces.

Apart from the pro-Marxist talk of Rene and his party newspaper, did Mancham have any evidence that the Soviets were behind the whole affair?

He said he had been warned by Gregory Mattson, U.S. Charge d'Affairs in the Seychelles, that there was "something to watch" and the Soviets may have been interested in the islands.

"I am personally convinced that the actions taken have been done with the agreement of the Soviet government and is part and parcel of the Soviet policy of controlling the Indian Ocean," Mancham said.

[There were some low-level Soviet visits a year or so ago, mostly by ships, and discussion of Soviet aid to build up the port at Victoria, but there has been no sign of any major Kremlin drive there, Washington Post correspondent Peter Osnos reported from Moscow.]

Mancham, asked if there was a member of the Central Intelligence Agency on the islands, said: "I have no idea that there are any CIA men in the Seychelles. Perhaps there should have been one."

Mancham had described his policy as nonaligned but he allowed the United States to operate an Air Force-manned satellite tracking station.

Above all, he had advertised the islands as a splendid place for parties, tourists and tax-evading funds. "We will try," he once said, "to pursue the philosophy of a small Indian Ocean Switzerland."

To that end, he imposed no controls over foreign exchange and attracted funds from more regulated Western nations. The new Marxist-leaning order is likely to end the Seychelles' role as a tax haven.

Not everybody in the Seychelles has as much fun as Jimmy, however. He himself estimated unemployment at 35 to 40 per cent. He could not give a more precise figure because no count is kept. Income per person is only about $500 a year.

A dozen young Seychellois gathered, hoping for news. One, Roger Decomarnoud, a garage owner, said: "If you're a rich man, you're for mancham.

If you're poor, you're for Rene." But his judgement was hotly disputed by several others.

"I've always been financially well off," Manchan explained. My father was one of the most successful businessmen in the Seychelles. He left me a considerable amount of money."

Mancham, who studied law in London and also went to schools in Paris and Geneva, has sizeable if unknown bank accounts outside his country.

The circumstances of his overthrown underlined that Commonwealth meetings, and other trips abroad, are always a testing time for government leaders in the Third World.

In 1971, Milton Obote of Uganda was deposed by Idi Amin while he was away in Singapore attending a Commonwealth leaders meeting. Others overthrown when abroad, although not while at a Commonwealth conference, include Nigeria's Yabuku Gowon in 1975, and Ghana's Kwame Nkrumah in 1966 and Kofi Busia in 1972.

With no army, navy or air force, Mancham must have thought it was safe to travel. But as one of his aides said: "Twentyfive men with sticks could stage a coup in the Seychelles."

From Nairobi, Washington Post special correspondent Roger Mann reported that diplomatic sources in the Kenyan capital felt it was too early to say what effects the Seychelles coup would have on American interests.

Beside the satellite tracking station and a small number of Peace Corps volunteers, there are virtuaily no American interests there.