Sir Mark Thatcher last night denied any involvement in an alleged plot to topple the government of oil-rich Equatorial Guinea.

The son of former Prime Minister Margaret Thatcher was charged yesterday with violating South Africa's anti-mercenary law in connection with an alleged coup attempt.

"I am innocent of all charges made against me. I have been and am cooperating fully with the authorities," he said.

"I have no involvement in an alleged coup in Equatorial Guinea and I reject all suggestions to the contrary," the 51-year-old said in a statement released by his spokesman, Lord Bell.

Sir Mark was brought before a magistrate's court in Cape Town before being released. Bail was set at two million rands (£175,000).

The charge followed his arrest early yesterday morning at his home in the wealthy suburb of Constantia.

The businessman was placed under house arrest and has until September 8 to post his bail.

In a bizarre development, Sir Mark's court appearance was delayed when he was robbed in a crowded holding cell. His shoes, jacket and mobile phone were taken, according to a court official.

He did not appear to have been injured and police were trying to recover the items.

Sir Mark, who inherited the baronetcy of his late father Sir Denis last year, could receive up to 15 years' imprisonment if convicted.

However, Equatorial Gui-nea's justice minister, Ruben Mangue, played down suggestions that the west African country may seek to extradite him.

Sir Mark's lawyer, Peter Hodes, said he was arrested on suspicion of providing financing for a helicopter linked to the alleged coup plot. "He will plead not guilty," he said.

The arrest came as trials take place in Equatorial Guinea and Zimbabwe over the plot, foiled in March, to overthrow the regime of President Teodoro Obiang.

It is claimed that Briton Simon Mann, an Old Etonian turned African mercenary, was the ringleader.

He was held in Zimbabwe, allegedly with a plane full of mercenaries on their way to overthrow the Equatorial Guinea government.

The alleged plotters were said to be hoping to exploit the country's massive oil reserves by installing their own leader, Severo Moto, currently in exile in Spain.

Mann is one of 70 defendants held in Zimbabwe while another 19 people are on trial in the Equatorial Guinea capital Malabo.

These include South African arms dealer Nick du Toit, another alleged key figure, who told the court yesterday he met Sir Mark before the alleged plot was foiled.

Du Toit, who faces the death penalty for his alleged role, said Sir Mark was interested only in purchasing military hardware that was not involved in the alleged plot.

He wanted to buy military helicopters for a mining deal with Sudan, he said.

Equatorial Guinea, pumping 350,000 barrels of oil a day, has become Africa's third-largest oil producer since offshore development began in the mid-1990s.

The country has accused Sir Mark, British and South African oil broker Eli Calil and other foreign financiers of funding the coup attempt.

Police in South Africa, with search warrants, raided Sir Mark's home in the suburb of Constantia shortly after 7am local time. He was wearing pyjamas when they arrived.

Police spokesman Sipho Ngwema said: "We have evidence, credible evidence, and information that he was involved in the attempted coup."

Sir Mark was in custody at his home while investigators searched his records and computers for evidence.

A spokeswoman for Baroness Thatcher said there was no comment on the situation. She is on holiday in the US and is due to return to the UK on Friday.