Mark Thatcher held over coup plot
Mark Thatcher, the son of the former UK prime minister, has been arrested at his home in Cape Town.
He has been detained by police investigating an alleged coup attempt in Equatorial Guinea.
A spokesman for South Africa's National Prosecuting Authority said he was held on suspicion of providing funding and logistical assistance.
Sir Mark, who has previously denied any involvement, will appear before Cape Town magistrates later on Wednesday.
Sipho Ngwema, the director of special operations for police elite anti-fraud unit the Scorpions, said they had "credible evidence" Sir Mark financed the coup attempt.
We are going to take all the documents we think are relevant to our investigation
"We refuse that South Africa be a springboard for coups in Africa and elsewhere," he added.
The 51-year-old father-of-two was arrested at his home in an exclusive suburb of Cape Town.
Mr Ngwema told BBC News Sir Mark was co-operating with the police, and their search was proceeding "unhindered".
"We are going to take all the documents we think are relevant to our investigation," he added.
The president of Equatorial Guinea, Teodoro Obiang Nguema Mbasogo, has reportedly accused Sir Mark of funding the alleged coup attempt.
National Prosecuting Authority spokesman Makahosini Nkosi said it was investigating charges of contravening the Regulation of Foreign Military Assistance Act.
"This is in relation to the possible funding and logistical assistance in relation to the attempted coup in Equatorial Guinea, " he said.
"We have conducted a search and seizure operation at his home in Cape Town."
Mr Nkosi told BBC Radio 4's Today programme: "I do not think we will be opposing bail - but there will be strict bail conditions that will prohibit him from leaving the country."
A spokeswoman for Baroness Thatcher said the former prime minister was on holiday in America and had not yet been contacted.
Court case
South African arms dealer Nick du Toit is accused of helping to organise a coup to overthrow the Equatorial Guinea president.
Mr du Toit went on trial with 13 other foreign nationals on Monday in the country's capital, Malabo.
The eight South Africans and six Armenians have been detained since March this year.
A 15th defendant died in prison. Both Amnesty International and Mr du Toit's wife allege the accused were tortured.
Seventy other accused mercenaries are on trial separately in Zimbabwe, where they were arrested on 6 March as they allegedly prepared to board a leased aircraft to launch the coup.
The alleged plot leader, former British SAS captain Simon Mann, an old Etonian turned leading African mercenary, has admitted trying to procure dangerous weapons - a charge which carries a possible 10-year jail sentence.
Sir Mark inherited his late father's hereditary baronetcy in 2003
http://news.bbc.co.uk/2/hi/africa/3596948.stm