Super League: London Broncos 40-36 Leeds Rhinos
London Broncos (8) 40
Tries: Atkins, Keyes 3, Dixon, McMeeken, Moore Goals: Drinkwater 6
Leeds Rhinos (24) 36
Tries: Watkins, Walters, Sutcliffe 2, Achurch 2, Hardaker Goals: Hardaker, Sutcliffe 3
London Broncos won their first match of the season at the 24th attempt as Challenge Cup finalists Leeds Rhinos suffered a shock defeat at The Hive.
The Broncos were 22 points behind early in the second half, but fought back to claim their first win in 11 months.
Half-back Joe Keyes scored three of their seven tries, while winger Kieran Dixon marked his return from a long-term knee injury with one of his own.
Leeds missed the chance to move above Wigan Warriors into second position.
A youthful Rhinos side, missing several first-team players ahead of their Challenge Cup final at Wembley against Castleford on Saturday, appeared in complete control when they led 30-8 early in the second period.
But the relegated Broncos, whose last victory came against Hull KR in September 2013, scored 20 unanswered points in the closing stages to register a shock win.
A win at last for London
The Broncos lost their first 23 Super League matches of the season and had not won since a 34-22 success at Hull KR in their final game of 2013
There was late drama to endure, however, as Leeds thought they had scored with the final play of the game - which would have been enough to earn at least a draw for the Rhinos - but it was ruled out by the officials.
Leeds started quickly, with Kallum Watkins touching down easily in the corner after two minutes and Josh Walters and Liam Sutcliffe also crossing early on.
The Broncos fought back with two scores in quick succession through Jordan Atkins and Keyes, but a powerful effort from Mitch Achurch and Zak Hardaker's excellent individual run gave the visitors a 16-point advantage at half-time.
Paul Aiton and Watkins set up Sutcliffe for his second try immediately after the break, but the hosts hit back once again with a superb team move finished off by winger Dixon, fit again after 10 months out, and Mike McMeeken's score under the posts.
Leeds second-rower Achurch ran in from 50 metres for his second try, but hooker Scott Moore kept London in the contest and two late tries from teenage half-back Keyes put the Broncos in front for the first time with six minutes to go.
Josh Drinkwater's penalty stretched their lead to four points in the closing seconds and Joey Grima's side held on for a long-awaited win.
London Broncos coach Joey Grima:
"I know Leeds had players missing today but they still had a quality team out there, and to come back like that is hard to believe.
"Given the season we have had, to produce a performance like that takes some doing.
"If I'm totally honest, I am pretty emotional about it all. There's a great atmosphere in the dressing room, although most of the players don't actually know the words to any victory songs."
Leeds coach Brian McDermott:
"I know people think we do not care about the league because of the Challenge Cup, but that is absolutely not true.
"In fact, if we miss out on top spot because of this game I will be very disappointed.
"There were individual errors out there today that are totally unacceptable - we did not play badly as a team, but there were too many mistakes from players.
"Credit to London though. They did not go away and that says a lot after the long, hard season they have had."
London Broncos: Farrar; Dixon, Mataitonga, Solomona, Atkins; Drinkwater, Keyes; Slyney, Moore, Wallace, Vea, McMeeken, Cook.
Replacements: Krasniqi, Griffin, Dollapi, Thomas.
Leeds Rhinos: Hardaker; Briscoe, Watkins, Handley, Hall; Sutcliffe, White; Singleton, Aiton, Bailey, S. Ward, Achurch, Walters.
Replacements: Kirke, Clarkson, Minchella, Golding.
Referee: Tim Roby