Millwall goalscorer Neil Harris battles for the ball with Leeds' Richard Naylor
Veteran striker Neil Harris struck a second-half winner to give Millwall a slender advantage after the first leg of their League One play-off semi-final against Leeds.
Harris controlled a low cross before creating a yard of space and drilling his strike past Casper Ankergren, who had earlier tipped a neat flick from the Lions striker on to the post.
The goal sparked a mini pitch invasion from a section of the home supporters and one fan was arrested on suspicion of assault after an incident involving Leeds keeper Ankergren.
But Millwall deserved their goal after a performance of tremendous desire and spirit that ensured their first play-off win in nine attempts.
Leeds showed in glimpses the form that had seen them lose just one of their final 15 games of the season, but they faded after the break and must now look to overturn the deficit during Thursday's return leg at Elland Road.
United striker Jermaine Beckford, with 34 goals this season, underlined the threat that he possesses in just the second minute, winning control of a long ball up field before forcing David Forde into action with a crisp strike.
It was probably the nearest we came to a goal in an opening half that was heavy on intent and endeavour but low on quality.
Millwall striker Jason Price, who went off injured after 15 minutes, headed the ball intelligently into the path of Andy Frampton but the full-back volleyed his effort wide. Veteran striker Harris replaced Price as the Lions persevered with their 4-3-3 formation and the substitute cut skilfully in from the right but saw his shot blocked.
Despite dominating periods of the half, the home team created few openings, while Fabian Delph with a long-range strike and Luciano Becchio with a limp header forced further saves from Forde.
David Martin and Robert Snodgrass had a meeting of minds while Neil Kilkenny and Adam Boulder tangled as emotions threatened to boil over, while Delph was perhaps lucky to escape with a booking after a scything challenge on Gary Alexander in first-half injury time.
Leeds were the first to threaten after the break, with Beckford's strike blocked by Frampton after the striker had linked-up brilliantly with Becchio to craft the opening.
But Millwall responded and finally started to open up the opposition.
Zak Whitbread went close from a free-kick, Harris' inventive flick almost defeated Ankergren and Alexander drilled a shot wide from 20 yards.
But it was Harris who found the net, controlling Alexander's low cross from the right before slotting into the bottom corner from eight yards after 70 minutes.
The lead might have been doubled shortly afterwards but Frampton could not keep his header on target.
Millwall manager Kenny Jackett:
"It's all set up and we know we've got to go to Yorkshire and play right to our maximum if we want to go through. These play-off games are very tight and this one is no different.
"It was a very good clean sheet. The back four kept a very good Leeds frontline quiet and it was key.
"It's very tight but there's no doubt that today has helped us slightly but we need a big performance to go through to Wembley."
Leeds manager Simon Grayson:
"I said that if we we're still in the tie after the game today then we can turn it round and we still are in the tie.
"We know now what we've got to do. We've got 90 minutes to turn a 1-0 deficit into a positive for us and we've got players that are capable of doing that.
"We've got 37,000 at Elland Road on Thursday night, which is going to be some atmosphere and that isn't going to get us through the game but it will help."
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