The members were toasting the achievement of England's spearhead and revelling in Australian woe. Everyone at the home of cricket believed they had witnessed one of the finest displays of fast bowling seen at the ground.
Indeed they had. But better was to come, in the final session of the day, when Glenn McGrath, with surgical precision, removed England's top five batsmen in a 31-ball spell which conceded only two runs. It was breathtaking stuff, and it made fools of those who were prepared to write the 35-year-old off as a spent force at the start of Australia's tour of England.
When McGrath knocked back Andrew Flintoff's off-stump with a vicious nip backer, the legendary seamer had claimed his third five-wicket haul in three Test matches at this venue, and England had been reduced to 21 for 5. And, as the home teams middle and lower order frantically sought their batting gear, Harmison's heroics began to feel like a distant memory.
On a dry pitch, which is already showing signs of inconsistent bounce, Australia's paltry total was now looking like a match-winning score. England still required 32 runs to take them past their lowest total against Australia - 52 at The Oval in 1948. But the debutant, Kevin Pietersen, and Geraint Jones did just this with a robust and skilful 58-run partnership.
McGrath, with the fantastic figures of 5-21 in 13 overs, was replaced by Brett Lee at the Pavilion End and the firebrand removed Jones with a well-directed bouncer which he spliced to the keeper. Ashley Giles should have followed, when he gloved a snorter to short leg, but the umpire, Aleem Dar, deemed Lee had put his foot over the front line.
Giles failed to make his good fortune count and trod on his wicket when facing the final ball of an exceptional day. Australia's players gathered in a huddle to congratulate each other as Giles, and the unbeaten Pietersen, trudged off to a disconsolate dressing-room.
England, on 92-7, are not out of this game. A few lusty blows from Pietersen this morning will quickly reduce the 98-run deficit. But Vaughan's side need to get close to Australia's first-innings total - and batting last on this pitch will not be easy.
In years to come Marcus Trescothick will probably look back on his dismissal with pride. There was little the left-hander could do about the off-cutter which jagged across him and took the edge of his bat. A jubilant Justin Langer, fielding at third slip, threw the ball in the air while his team-mates engulfed their hero.
In the build-up to the Test, ingenuous members of Vaughan's side had encouraged England supporters to get stuck in to the Aussies, but it was magnificent to see Lord's rise to its feet to salute McGrath's 500th Test wicket.
And it did not take long for McGrath to start edging his way to 600, as Andrew Strauss nibbled at a good-length ball and was caught by Shane Warne at first slip.
Before each series the New South Welshman publicly names the opposition batsmen he is after, and on this tour it has been Strauss and Vaughan. With Strauss gone, McGrath turned his focus to Vaughan - and in his seventh over he got his man. The reaction of the England captain suggested that the ball kept low, and it did a little, but not as much as he thought. It clipped the top of his off stump.
Ian Bell chopped on in McGrath's next over, and when Flintoff departed a sombre mood had descended on Lord's.
Harmison may not yet have the control of McGrath but he was a fearsome proposition with the new ball. The Durham paceman showed his intent with the second ball of the day, which struck Justin Langer painfully on the right elbow. But Langer and his opening partner Matthew Hayden, who was struck on the head by Harmison, regained their composure and set about making the most of Ricky Ponting's decision to bat.
Matthew Hoggard struggled to control a swinging new ball on an overcast morning but got the last ball of his fourth over on the spot, and it knocked back Hayden's off-stump.
Ponting became the third Australian to call for assistance when he was struck on the visor by another brutish delivery from Harmison. The blow put a one-inch gash in the right cheek of the Australian captain, but after five minutes of treatment he batted on.
But the disorientated Ponting did not last long, and Harmison showed no mercy when he produced a delightful leg cutter to dismiss him.
Flintoff replaced his close friend at the Pavilion End and with his fourth ball in Ashes cricket he claimed the scalp of Langer. The Australian opener had been in positive mood but an ill-advised pull at the England all-rounder proved costly.
Simon Jones made an even quicker impact than Flintoff when he took the wicket of Damien Martyn with his first ball, and Michael Clarke soon followed when he was trapped in front by the fired-up Welshman. At lunch Australia, amazingly, were on 97 for 5.
Adam Gilchrist has consistently taken Australia out of perilous positions with his fearless and unique style, and Australia's plight failed to stop him going for his shots. The left-hander's 19-ball innings of 26 contained six boundaries, but his fun ended when he edged Flintoff through to Geraint Jones. Simon Katich and Shane Warne battled until Harmison returned. Warne was bowled behind his legs, Katich was caught pulling, Lee was caught off an inside edge and Jason Gillespie trapped plum in front.
Harmison will need to do the same again today if England are to end 71 years of pain against Australia at the home of cricket.