Mr Clinton was expected to spend all afternoon - around four hours - answering questions from the independent prosecutor, Kenneth Starr, and his team in what had been widely billed the end-game of his seven-month criminal investigation.
The investigation, which has polarised American public opinion and led to talk of a personal vendetta between the President and the prosecutor, concerns allegations that Mr Clinton lied under oath about an affair with a former White House trainee, Monica Lewinsky, and put pressure on her to lie about it also.
Ms Lewinsky, who gave her own testimony 10 days ago after negotiating complete immunity from prosecution, reportedly admitted to an affair, but denied instructions from Mr Clinton to lie.
There was speculation from early yesterday that Mr Clinton would go on television some time during the evening to present his case directly to the American public. The White House spokesman, Mike McCurry, tried to dampen speculation by insisting that no decision would be taken until after Mr Clinton had completed his testimony.
He did confirm, however, that Mr Clinton's personal lawyer, David Kendall, would issue a brief statement.
Mr Clinton began his testimony at 12.59pm. He had his three personal lawyers with him and the proceedings were relayed live, amid ultra-high security, to the grand jury at the Washington courthouse 10 blocks away.
As part of the terms for his agreement to testify, Mr Clinton had been permitted to testify on closed-circuit television from the White House rather than submit to the indignity of attending the courthouse in person.
The atmosphere around the White House was fraught. In the rabbit-warren of the press room, luminaries of the White House press corps disputed the merits of Mr Clinton's case and his survivability. Have presidents always lied and got away with it?
Are the Nineties more invasive or less forgiving than the Sixties? Should a president serve as a national paragon of morality?
In a poignant reminder of what might have been, copies of Mr Clinton's original agenda for Monday 17 August were still in reporters' pigeon-holes: he was due to have left Washington at 7.30am for his holiday on Martha's Vineyard.
In the main part of the White House, the presidential offices and quarters were reported to be quiet as the President held last-minute meetings with his advisers, among them his Chief of Staff, Erskine Bowles, and his National Security Adviser, Sandy Berger.
The previous evening he prayed with the civil-rights leader the Rev Jesse Jackson, who has been something of a spiritual adviser to Mr Clinton since the Lewinsky allegations broke in January.
Mr Jackson, effectively confirming that Mr Clinton would amend his earlier denials to admit some sort of sexual activity with Ms Lewinsky, told reporters that the President was "embarrassed" by the case, but that his wife, Hillary, and daughter, Chelsea, were standing by him and that his marriage would survive.
"Hillary has had to face the humiliation of it all. But then, she is mature and they are in love and their marriage will survive this," he told the CNN channel.
Mr Jackson's comments seemed intended to counter earlier reports that the President was alternately depressed and angry, and that relations between the First Couple were tense.
A plethora of reports over the weekend had said that Mr Clinton was preparing to admit to an "inappropriate" relationship of some kind with Ms Lewinsky, while insisting that he had not perjured himself in earlier denials.
The multitude of legal and political commentators in Washington inclined to the view that, while legally tenable, this could prove difficult.
Over the months successive Clinton loyalists, including his communications director, Ann Lewis, a senior adviser, Rahm Emanuel, and the White House counsellor, Paul Begala, have all told the press, in variations of Ms Lewis's words, that "sex is sex, even in Washington", and that Mr Clinton's denials meant what they said.
The President's most celebrated denial - on television to the American public - that he did "not have sex with that woman, Ms Lewinsky", could therefore prove his greatest liability.
Despite fevered speculation over the past week, however, Mr Clinton's popularity ratings have held up well. While a growing majority of people (70 per cent) believe that he did have a sexual relationship with Ms Lewinsky and lied about it, the same proportion - the highest on record - say they approve of his performance as president.
Each day brings forecasts - the latest from Andrew Kohut, of the Pew Research Centre - that these figures could rapidly turn against Mr Clinton. But they have not done so yet.
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