New York's mayor-elect Zohran Mamdani - who described himself as "Trump's worst nightmare" - said he's shown how the Democrats can defeat the US president as he celebrated victory in the city's election.
Sky's US partner network NBC News projected he had won with 60.2% of votes counted, and his closest rival - the former state governor Andrew Cuomo - conceded not long after.
Mr Mamdani - who will be the first Muslim, first South Asian, and youngest person in a century to be elected as the famous city's mayor - used a victory speech to rapturous supporters in Brooklyn to poke at Mr Cuomo and throw down the gauntlet to Mr Trump.
"I wish Andrew Cuomo only the best in private life," he said of the man he beat in the contest to be the Democratic nominee, "but let tonight be the final time I utter his name as we turn the page on a politics that abandons the many and serves only the few."
Turning his fire on Mr Trump, who had backed Mr Cuomo and threatened to withhold federal funding from New York if the democratic socialist won, Mr Mamdani said: "If anyone can show a nation betrayed by Donald Trump how to defeat him, it is the city that gave rise to him.
"This is not only how we stop Trump. It's how we stop the next one. So Donald Trump, since I know you're watching, I have four words for you: 'Turn the volume up'."
On Truth Social, the US president said in response: "...AND SO IT BEGINS!"
'The impossible made possible'
More than two million people voted in this year's general election, according to the New York Board of Elections. In 2021, the turnout was just under 1.15 million.
Mr Mamdani ran on a progressive platform with ambitious policies that critics have suggested won't be affordable, including state-run grocery shops, "fast and free" buses, and frozen rents on some housing.
He has vowed to raise taxes on the rich to help pay for his plans, and called himself "Donald Trump's worst nightmare" in August.
"We won because New Yorkers allowed themselves to hope that the impossible could be made possible," he told supporters late on Tuesday.
Mr Mamdani will be sworn in on New Year's Day.
The projected win comes after one of the most colourful races in years and looks set to have a profound effect on US national politics.
Read more:
Who is Zohran Mamdani?
How did we get here?
After the current Democratic mayor, Eric Adams, announced he would run as an independent after a corruption scandal, Mr Cuomo emerged as frontrunner to take the party's nomination.
He was looking to rebuild his career after resigning as governor of New York State in 2021 amid allegations of sexual harassment.
But in a major upset, relative outsider Mr Mamdani took the nomination instead, and became the favourite for mayor. Mr Cuomo then chose to run as an independent.
On the eve of the election, Mr Trump said Mr Mamdani - who is known for left-wing policies and campaigning on issues like affordable housing - would bring "disaster" to the city.
He encouraged backers of Curtis Sliwa, the Republican candidate who trailed in polling, to transfer their votes to Mr Cuomo instead.
Big name backers
While some Democrats avoided endorsing Mr Mamdani, he won support from progressives like Bernie Sanders and Alexandria Ocasio-Cortez.
His fans in the UK include former Labour leader Jeremy Corbyn, who helped canvass for the 34-year-old by phone banks, and new Greens leader Zack Polanski.
Mr Polanski has drawn some comparisons to Mr Mamdani for his left-wing policies and use of social media.
What has Cuomo had to say?
In his concession speech, Mr Cuomo thanked his supporters - who he called "superstars" and "New York patriots" - before saying he was "proud" of his campaign.
"This campaign was the right fight to wage, and I am proud of what we did and what we did together," he said.
Mr Cuomo touted his campaign of "unity," before saying New Yorkers "will not tolerate any behaviour that fans the flames of antisemitism".
He had regularly attacked Mr Mamdani for his past comments about Israel, and support of Palestine, during the campaign.
Boos rang out as Mr Cuomo ended by congratulating Mr Mamdani, prompting him to say: "This is not us... Tonight was their night."
Democrats win key governor races
New York's mayoral race was one of four elections on Tuesday serving as a bellwether referendum on Mr Trump's second term as president.
Democrats Abigail Spanberger and Mikie Sherrill won their races to become state governors in Virginia and New Jersey, respectively.
The US president's first reaction to the Democratic wins across the country came on Truth Social.
He repeated claims from "pollsters" that said the US government shutdown, and the fact he "wasn't on the ballot", were the two reasons Republicans lost.
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