The British mercenary Simon Mann today told an Equatorial Guinea court that Mark Thatcher, the son of the former prime minister, was a committed member of the group that organised the attempted coup in the oil-rich west African state in 2004.

Giving his first detailed account of the planning for the coup, in a clear and confident voice he said Thatcher "was not just an investor, he came completely on board and became a part of the management team". He said Thatcher had provided $350,000 (£178,000) in funding for the coup.

In further testimony, Mann claimed that Spain and South Africa, with the endorsement of the South African president, Thabo Mbeki, had supported the plot. By January 2004, two months before the attempted coup was put into action, it was, Mann said, "like an official operation. The governments of Spain and South Africa were giving the green light: 'You've got to go, you've got to do it.'"

Spain, he said, was prepared to recognise the new government the day after the coup and to send in large numbers of military police. Outside the court, the Spanish ambassador, Javier Sangro, said he had no comment.

Senior members of the Equatorial Guinea army, police and cabinet were also implicated, Mann said, and he was given details of President Teodoro Obiang's daily movements and his health problems.

From the Pentagon in Washington, and from the CIA and the big US oil companies, came tacit approval for regime change, according to Mann. Their message, conveyed by one of his colleagues who went to Washington discreetly to test the waters, was that "the political situation in Equatorial Guinea was very unsatisfactory and very dangerous and that a well-conducted change of government would be welcomed", he said.

At the end of his four-hour examination, with the help of an interpreter, Mann made his plea for clemency after it was recommended yesterday he serve a prison sentence of 32 years.

"I am very sorry for what I did in 2003 and 2004. I am very sorry for what I've done. I am also very happy that we failed, that it didn't work, especially now that I am here and I've met you all."

Referring to his incarceration in Zimbabwe after being arrested with a plane-load of mercenaries en route to Malabo, the Equatorial Guinea capital, he said: "I've been in prison for four years and I'm not the same man that I was."

Although he delivered his story with flashes of humour, emphatic gestures and at times almost with relish, he was apparently suffering from a hernia, gripping his lower abdomen as he stood before the attorney general. After two hours, the presiding judge allowed him to sit down.

The name of Lady Thatcher's son cropped up when he was asked about planning meetings in South Africa. Both Mann and Thatcher lived there at the time. Thatcher, who now lives in a gated estate in southern Spain, accepted a plea bargain from the South African authorities in 2005 after he admitted helping to finance a helicopter that he suspected "might be used for mercenary activities".

Thatcher was fined $450,000 and given a four-year suspended sentence. He left the country shortly afterwards and has refused to elaborate on his role. The Equatorial Guinea government has tried to get an international warrant for his arrest.

In the months leading up to the coup, Thatcher attended many meetings, Mann said, "because at this stage he came on board".

Mann took Thatcher to the Chelsea home of Ely Calil, the Lebanese businessman who is alleged to have been the main financier of the plot. He named the management board as Calil, himself, a London property developer, Thatcher and a Lebanese colleague of Calil who lives in Beirut.

Although Calil currently rents a mansion in Hampstead, he is thought to be in Beirut. He has always denied involvement in the coup plot but has never given an interview about the allegations.

The plan involving the helicopter was dropped. But Thatcher's money was used to buy a small plane that would transport the new provisional president, Severo Moto, from his opposition exile in Spain to Malabo via the Canary Islands.

Mann said Calil had initially asked him, in May 2003, to assassinate Obiang and to launch a guerrilla war or a coup. "I said I would not do it, on ethical grounds, and also because it was a very stupid thing to do," Mann said. He accepted he was doing the job for money – said to be $15m - but he claimed he was sympathetic to the story he was told that oil money was not reaching the people. "I believed it was right."

At the beginning of his testimony, in answers to questions from the prosecutor, Jose Olo Obono, the attorney general, Mann agreed he had been well treated since his extradition from Zimbabwe in February. Asked if he agreed with the proposed sentence of 32 years, he replied: "I don't agree with that, no." And he added a long and plaintive: "Please."