London ? South African police Wednesday seized Mark Thatcher, the scandal-plagued son of former Prime Minister Margaret Thatcher, on charges he took part in a plot with mercenaries to overthrow the dictatorship of oil-rich Equatorial Guinea.

An anti-corruption unit of the South African police raided Thatcher’s luxurious villa in the posh Cape Town suburb of Constantia at 7 a.m., surprising the 51-year-old businessman and former race-car driver in his pajamas.

Thatcher appeared before a magistrate and was put under house arrest and given until Sept. 8 to post bail of $300,000. His lawyer insisted he was innocent and cooperating with authorities.

A prosecutor said that Thatcher faces a maximum penalty of 15 years in prison if convicted.

“We have evidence, credible evidence, and information that he was involved in the coup,” police spokesman Sipho Ngwema said. “We refuse that South Africa be a springboard for coups in Africa and elsewhere.”

Thatcher, 51, who inherited the title of baronet after his father, Sir Denis Thatcher, died last year, has been in the headlines frequently — and seldom in a complimentary light — since his mother became prime minister in 1979.

Over the years, he has been accused of trading on his mother’s name and connections to become a multimillionaire in a series of murky business dealings. “Mark could sell snow to the Eskimos and sand to the Arabs,” his mother once reportedly said of him.

In Britain, news of Thatcher’s arrest immediately prompted worries about how it would affect Lady Thatcher, who is approaching her 79th birthday and whose health has been of concern since she suffered a series of minor strokes in recent years and endured the death of her husband in June 2003.

In the United States on a vacation, and due to return to England on Friday, Lady Thatcher had no immediate statement, a spokesman said.

Mark Thatcher is the neighbor in South Africa and close friend of Simon Mann, a former British SAS officer accused of leading an alleged attempted coup aimed at Equatorial Guinea’s longtime strongman president, Teodoro Obiang.

Mann was arrested in Harare, Zimbabwe, with 70 men and arms, en route to Equatorial Guinea.

Mark Thatcher, left, the son of former British Prime Minister Margaret Thatcher is escorted by police outside his home in Cape Town, South Africa. Thatcher was arrested Wednesday on allegations he was involved in a plot to overthrow the government of oil-rich Equatorial Guinea.