Orient put in a performance that would surely have been enough to get at least a draw against London and promotion rivals Brentford on any other day but were unable to find a response to the visitors' first half goal in front of the biggest home crowd of the season.
After dominating the opening period, Orient found themselves one-down at the break after cruelly conceding in the final seconds of injury time when Adam Forshaw and Marcello Trotta linked up well before the latter fired in from 20 yards.
It was a killer blow to the O's but it looked as though their luck had turned when, just four minutes into the second half, Bees defender James Tarkowski was sent off when he was shown a second yellow card for a poor challenge on Romain Vincelot.
However, despite continuing to have the better of the game and going agonisingly close to finding the back of the net on several occassions, Orient were unable to grab the equaliser they so deserved.
Always a huge occasion for both clubs, the London derby between Orient and Brentford has taken on even greater significance this season with both sides fighting to secure automatic promotion the Championship.
And that was demonstrated perfectly by the sell-out crowd at Brisbane Road – the biggest attendance since fellow Londoners Arsenal visited in the FA Cup in 2011 – as well as the Sky Sports cameras coming to E10 to witness this top-of-the-table clash.
Both sides came into the fixture having recovered well from the setbacks they received in February - Orient bounced back from three straight defeats to take 14 points from the last six games which took them back into the automatic promotion places, while Brentford had gone three games unbeaten after League 1 leaders Wolves thumped them 3-0 at Griffin Park last month.
Russell Slade made just one change to his starting XI after a strong away performance at Port Vale in midweek which saw the O’s run out 2-0 winners; Kevin Lisbie returning to partner David Mooney upfront, with Chris Dagnall the unfortunate man to make way.
Brentford overcame Tranmere Rovers by the same scoreline on Tuesday but Bees manager Mark Warburton made two changes to his side this afternoon, with first team regulars Forshaw and Trotta replacing Will Grigg and Jake Reeves.
The O’s were much the better side from the moment they created the first real chance of the game just after the 10-minute mark.
The home side broke away having defended a Brentford corner well, with Mooney feeding the ball into strike partner Lisbie and continuing his run to send a cheeky dink narrowly over the crossbar after Lisbie had cut inside the penalty area.
The Irishman might have been better to put his foot through the shot and at least force David Button in the Bees goal in to action, but it would have been a contender for goal of the season had his chip found the net.
This triggered a great spell for Orient, including a drive from winger Dean Cox which narrowly missed the target and series of corners and free-kicks in threatening positions.
Despite the O's dominance, the home side had to call Eldin Jakupovic into action on 23 minutes when George Saville sent Donaldson through on goal but the Bosnia keeper was out quickly to block the striker’s shot with his left palm.
Cox was Orient's liveliest attacking player all afternoon and he came close again just before the half-hour mark when superb forward play from Mooney and Lisbie allowed him a clear sight of goal only for Button to save his well-struck effort at the near post.
Slade and his backroom staff had ordered their men to press Brentford – a side renowned for playing possession-based football – high up the pitch, and it was a tactic that appeared to be working as the visitors’ limited threat was coming in the form of counter-attacks only.
As the sun shone down in E10, the heat of the battle was coming to the fore and a fiery coming together between Gary Sawyer and Adam Forshaw on the left led to a mass brawl, during which the O's left-back picked up a booking together with Bees striker Donaldson who had raised his hands and could have been given his marching orders on another day.
The game took a scrappy turn after this and it looked as though half-time would arrive before either side managed to find another opening however Brentford worked the ball up the pitch in the dying seconds of stoppage time and took the lead against the run of play.
Forshaw's neat flick with the outside of his right boot found itself at the feet of Trotta who fired into the bottom left from 20 yards out - sending the 2,597 away fans into raptures.
The second half started in just as dramatic fashion as the first one had ended. Just four minutes in, Bees defender Tarkowski – already on a yellow card after an unnecessary challenge on Scott Cuthbert in the first half – flew into a tackle on Vincelot inside the O’s half and was shown a second booking by referee Robert Madley, elevating the atmosphere to a new level.
The visitors’ discipline continued to let them down with Forshaw giving away a free-kick which almost led to an O’s equaliser five minutes later, seeing the Bees midfielder pick up a yellow card in the process.
The game might have been a fiery affair but it wasn’t lacking in skill and flair, including another flick from Vincelot inside the opposition 18 yard box to tee up Lisbie, who will feel he should have at least hit the target with his first-time effort.
Orient came agonisingly close to grabbing the equaliser their performance deserved with just over 15 minutes to play when a great ball back across to box from Cox found its way to Odubajo – who had switched to right-back after the introduction of Shaun Batt on the wing – but he couldn’t find his 11th goal of the campaign, firing in to the legs of Button.
The tempo of the game remained relentless for 20 minutes after the Bees went down to 10 men but, having clawed their way through that spell, the away side managed to get men behind the ball and limit the opportunities for Orient.
The O’s also looked tired after dominating an entire game and seeing no reward for their efforts, but they still had chances to find an equaliser.
A cross-cum-shot from substitute Chris Dagnall in the closing 10 minutes was deflected inches wide of the Bees left-hand post and Batt’s header from the resulting corner-kick delivery was stopped on the line by Alan McCormack.
That was to be the final time Orient threatened the Brentford goal despite never giving up hope, and the visitors could have snatched another themselves deep into injury time when Alan Judge’s shot cannoned back off Jakupovic’s near post.
As it was, the score remained 1-0, meaning today’s opponents leapfrogged the O’s into second place in League 1 as the season enters its most crucial period.
In the lead up to this game, Russell Slade noted that results are more important than performances at this stage of the campaign but the way Orient played today suggests they still have what it takes to push the automatic promotion race right to the wire.
ORIENT: Jakupovic, Omozusi, Clarke (C), Cuthbert, Sawyer (Batt, 70), Odubajo, James (Bartley, 89), Vincelot, Cox, Mooney, Lisbie (Dagnall, 75)
SUBS NOT USED: Larkins, Ness, Lasimant, Simpson
BOOKINGS: Sawyer (40), Omozusi (69)
BRENTFORD: Button, Forshaw, Craig (C), Douglas, Donaldson, McCormack, Saville, Judge Bidwell, Tarkowski, Trotta (Dean, 51)
SUBS NOT USED: Lee, Grigg, Dallas, Reeves, Yennaris, Adams
GOALS: Trotta (45+2)
BOOKINGS: Tarkowski (18, 49), Donaldson (40), Forshaw (53), Judge (86)
SENT OFF: Tarkowski (49)
REFEREE: Robert Madley
ATTENDANCE: 8,335 (2,597)