Labour secured a surprise victory in Hove and Portslade.
Celia Barlow was elected with a tiny majority of 420 votes ahead of Conservative Nicholas Boles who had been favourite to win.
Ms Barlow's election was greeted with jeers as she accepted her victory at Hove Town Hall in the early hours of this morning.
For about an hour there was speculation there would have to be a recount because it was so close.
But after the vote bundles were checked, the returning officer was ready to announce the results.
Ms Barlow becomes Brighton and Hove's first woman MP.
She said: "I am delighted to be part of a historic night for the Labour Party - the first time we have won three general elections in a row."
Ms Barlow paid tribute to her opponent Mr Boles and said his positive campaign contrasted with the negative one by Tory leader Michael Howard.
When Ms Barlow tried to pay tribute to Prime Minister Tony Blair for helping out with Labour's campaign in Hove and Portslade, some of the crowd began to jeer and boo.
The announcement of the results descended into farce as Independent candidate Bob Dobbs climbed on to the stage and began shouting. Silent Majority candidate Richard Franklin began shouting too.
No security officers or police appeared to intervene and the losers' speeches were abandoned.
Mr Boles, 39, spent thousands on his campaign to persuade Tory voters to return to the fold.
Many predicted he would win and his defeat will be a bitter blow to local Conservatives. He refused to speak to The Argus after the result.
Lib Dem candidate Paul Elgood, a Brighton and Hove city councillor, was third with 8,002 votes, more than double the party received at the last general election.
He said: "It's a fantastic result. We are a strong fighting force in Hove and Portslade."
Ms Barlow told the crowd: "I'm honoured to have the chance to represent you whether you voted for me or not and I will work tirelessly." She was jeered as she left the stage surrounded by security.
Elected: Celia Barlow (L) 16,786.
Not elected: Nicholas Boles (C) 16,366, Paul Elgood (LD) 8,002, Anthea Ballam (GP) 2,575, Stuart Bower (UKIP) 575, Patrick O'Keeffe (The Respect Party) 268, Bob Dobbs (I) 95, Richard Franklin (Silent Majority Party) 78, Brian Ralfe (I) 51.
Turnout: 64.3 per cent.
Majority: 420.
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