Former British Prime Minister Margaret Thatcher's son, Mark, was arrested in South Africa early Wednesday in connection with the alleged plot to overthrow the government of oil-rich Equatorial Guinea.

Members of the elite South African police unit, known as the Scorpions, arrived at the door of Mark Thatcher's luxurious home in a Cape Town suburb early Wednesday morning to arrest him under South Africa's tough anti-mercenary laws.

Police say they have evidence that Mr. Thatcher helped to fund a failed coup attempt against Teodoro Obiang Nguema, president of Equatorial Guinea, Africa's third-largest oil producer.

The arrest came as trials are under way in Zimbabwe and Equatorial Guinea in the alleged coup plot. Many of the defendants have connections to the former apartheid South African military and to a mercenary group called Executive Outcomes.

The 51-year-old Mr. Thatcher has been mentioned in local and British media as having connections with one of the alleged ringleaders, British-born Simon Mann, who is on trial in Zimbabwe. President Nguema has also accused Mr. Thatcher and an unnamed former British minister of being involved in the plot in an interview with the French publication Jeune Afrique.

But in the Equatorial Guinea trial Wednesday, one of the alleged coup plotters denied Mr. Thatcher's involvement. South African Nick du Toit, a mercenary and arms dealer, testified he had been introduced to Mr. Thatcher by Mr. Mann, but that it was part of a normal business transaction to purchase military helicopters for a mining deal in Sudan.

This is not the first time that the son of Mrs. Thatcher, who served as British prime minister from 1979 to 1990, has made the headlines. In 1982, he disappeared in the Sahara while competing in the Dakar rally, causing his mother, nicknamed the "Iron Lady," to break down in public. He and his co-driver were found five days later unharmed in their disabled car.

South African police have not indicated whether he could be extradited to Equatorial Guinea where the death penalty is still in place and the government has been accused of torturing the alleged plotters.