With human rights activists and former Chilean dissidents packing the ornate chamber, the lords read out their individual verdicts like boxing judges announcing scorecards in a championship fight.
When Lord Hoffmann rose to give the deciding judgment, upholding the general's arrest, the ramifications reverberated around the world.
Human rights campaigners heralded the ruling as a landmark in British legal history.
A human rights lawyer, Geoffrey Robinson QC, said: "The ruling will make the torturers of the 21st century tremble. In the past, diplomats were trusted with making arrangements for torturers to leave the scenes of their crimes with amnesties in their pockets and their Swiss bank accounts intact."
Helen Bamber, director of the Medical Foundation for the Care of Victims of Torture, said: "This is a milestone in the battle against those who abuse human rights with impunity. "
The ruling - which drew on existing human rights conventions - has major implications for Britain's international relations. Although it is unlikely to lead to a rush of prosecutions against heads of state with poor human rights records, it is likely to make such leaders reluctant to risk visits to Britain.
After the verdict, Lord Steyn, one of the judges who upheld the appeal, said that the crimes of which General Pinochet was accused should no more be categorised as "acts undertaken in the exercise of the functions of a head of state" than "murdering his gardener".
He said that in the High Court, the Lord Chief Justice, Lord Bingham, had wrongly concluded that "even in respect of acts of torture" state immunity for a former head of state would prevail.
He also disagreed with Mr Justice Collins' High Court comment that: "There is in my judgment no justification for reading any limitation based on the nature of the crimes committed into the immunity which exists."
In his written judgment, Lord Steyn said: "It is inherent in this stark conclusion that there is no or virtually no line to be drawn.
"It follows that when Hitler ordered the 'final solution' his act must be regarded as an official act deriving from the exercise of his functions as head of state."
The crucial judgment was that of Lord Nicholls of Birkenhead, who declared that no one, not even a head of state, could get away with certain abhorrent crimes. "International law has made plain that certain types of conduct, including torture and hostage-taking, are not acceptable conduct on the part of anyone. This applies as much to heads of state, or even more so, as it does to everyone else. The contrary conclusion would make a mockery of international law."
Lord Nicholls' findings were mirrored by Britain's two liberal South African-born judges, Lord Steyn and Lord Hoffmann, to give a 3-2 verdict in favour of the appeal.
The chairman of the panel, Lord Slynn, had been the first to give judgment, finding against the appeal by the Crown Prosecution Service on behalf of the Spanish authorities seeking extradition. In one sentence he made known his views, saying: "I would hold that the respondent as a former head of state is immune from arrest."
Next came Lord Lloyd who also gave a dissenting judgement dismissing the appeal in favour of General Pinochet.
"It has not been suggested that he was personally guilty of any of the crimes of torture or hostage-taking in the sense that he carried them out with his own hands," he said.
"What is alleged ... is that he organised the commission of such crimes ... I do not see how in these circumstances he can be treated as acting in a private capacity."
At that point, the general needed just one of the remaining three judges to find in his favour and he would be whisked to his waiting Chilean air force jet at RAF Brize Norton, in Oxfordshire.
But after Lord Nicholls's verdict, Lord Steyn, who had left the apartheid regime in South Africa to live in Britain in 1973, said simply: "General Pinochet has no immunity whatever." Lord Hoffmann then announced that he concurred.