The new MP for Peterborough said the Brexit Party's "politics of division" was beaten by her own "politics of hope".
Labour's Lisa Forbes took aim at her closest rivals after triumphing by 683 votes over Brexit Party candidate Mike Greene following a hard-fought by-election campaign.
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Delivering her victory speech at the KingsGate Conference Centre in Parnwell, Ms Forbes said: "There have been many in recent months who have claimed democracy in this country is broken, but you have proven them wrong. A democracy only works when people make their voices heard for the issues that matter to them and their families.
"I fought this campaign on safer streets, more investment for our schools and a better deal for Peterborough. The fact that you have turned up in such large numbers is a testament to the fact that many here are desperate to see Peterborough change for the better.
"Despite differing opinions across our city, the fact that the Brexit Party have been rejected here in Peterborough shows the politics of division will not win.
"This is a result for every community in Peterborough. Tonight's victory is significant because it shows the politics of hope can win, regardless of the odds."
Ms Forbes, who thanked council staff for running the election, her opponents and her own team, said she wil not let down voters in a nod to former MP Fiona Onasanya, whose recall after being jailed for perverting the course of justice triggered the by-election.
Ms Onasanya was voted in as a Labour MP by just 607 votes in 2017 but had the party whip removed the day after her conviction, which was related to lying over speeding points.
Ms Forbes added: "The people of Peterborough have placed their trust in me, and I will not let you down. I promise to fight tirelessly for Peterborough to make it a city we can all be proud of, and that work starts now."