CAPE TOWN, South Africa – Mark Thatcher, the son of the former British prime minister, Margaret Thatcher, was planning to leave his home in South Africa for America when he was arrested in connection with an alleged coup plot, a police spokesman said yesterday.

Mr. Thatcher was arrested Wednesday and charged with helping finance what authorities say was a foiled plot to overthrow the president of oil-rich Equatorial Guinea.

Police in South Africa, where Mr. Thatcher has lived since 1995, have information that the 51-year-old businessman had put his $3.3 million home up for sale and had booked flights for his American wife and two children to Dallas, police spokesman Sipho Ngwema said. The children had already been enrolled in schools there.

“The flights were booked for Monday, and we believe Thatcher would have followed them. If you move your family and sell your property, it follows that you would be leaving the country, too,” Mr. Ngwema said.

Mr. Thatcher is under house arrest until he posts bail of $300,000. He is charged with violating the country’s Foreign Military Assistance Act for allegedly paying for a helicopter to assist in the coup plot. In a statement released yesterday, Mr. Thatcher maintained his innocence, the South African Press Association reported.

“I have been and am cooperating fully with the authorities in order to resolve the matter. I have no involvement in an alleged coup in Equatorial Guinea, and I reject all suggestions to the contrary,” the statement said.