Leyton Orient's teenage goalkeeper Glenn Morris pulled off a wonder save four minutes into stoppage time to ensure victory in a full-blooded derby duel.
Morris tipped a Neil Jenkins header over the bar just as it seemed that Southend's second- half pressure was set to earn them a point.
In the end, Orient's goals in the first and last minute of the first half were enough to stretch their unbeaten run to four games.
After the game Orient manager Paul Brush said: "We played with terrific tempo and desire in the first half but I didn't enjoy going to the 95th minute and seeing our goalkeeper having to make a tremendous save.
"There is a good buzz around the place at the moment, it's been a good week for us." But an unhappy Southend boss Rob Newman said: "I'm disappointed and I'm going to ruffle a few feathers and see if I can bring a few new players." Orient could not have dreamed of a better start as they took the lead after 56 seconds.
Mathew Lockwood fired in a corner from the left and when the ball wasn't cleared on- loan defender Ezomo Iriekpen headed home.
Orient could have stretched their lead as Kevin Nugent and John Martin both flashed headers just wide of the post.
As Orient continued to press Southend goalkeeper Darryl Flahavan brilliantly parried Nugent's close-range header before Orient stretched their advantage on the stroke of half time.
Donny Barnard's long ball was headed into his own net by Phil Whelan as Nugent put the defender under pressure.
After the break Newman made two changes and as Southend forced their way back into the game Morris made a super save from the dangerous Jay Smith.
Southend substitute Barrington Belgrave pulled a goal back on 73 minutes when Orient failed to clear, but his joy turned to despair when he was later sent off.
But even with ten men Southend continued to battle and Morris had to make his injury- time save to deny them a point