Andrew Cuomo has conceded in the New York City mayoral Democratic primary to state assemblyman Zohran Mamdani in a stunning political upset.

Cuomo, the state's former governor, was attempting to pull off a political comeback after resigning from office in 2021 amid a sexual harassment scandal.

In a speech to supporters, Cuomo said Mamdani - a 33-year-old democratic socialist - had "won" the primary race and that "we are going to take a look and make some decisions".

"Tonight is his night," Cuomo, 67, said. If elected, Mamdani would be the first Muslim and Indian American to lead the nation's largest city.

The primary in staunchly liberal New York is likely to determine who becomes mayor in November's election.

The contest was being watched as a litmus test for the Democratic Party as it seeks to hone its messaging after election losses last November that saw President Donald Trump's Republicans win the White House and both chambers in Congress.

Results on Tuesday night showed Mamdani with a commanding lead, but falling short of the 50% threshold needed to win outright.

Cuomo's concession was unexpected because counting looks likely to continue next week under the ranked choice system, which allowed New Yorkers to pick up to five candidates in order of preference.

The former governor's loss marks the "biggest upset in modern NYC history," Trip Yang, a political strategist, told the BBC.

"A massive win for Zohran Mamdani that shows that when Donald Trump is President, New York Democrats want to see their leaders fight with enthusiasm and courage, and that's what Zohran showed voters."

In an interview with the New York Times, Cuomo said he was still examining whether he would run in the general election in November on the independent line.

"I said he won the primary election," Cuomo told the outlet. "I said I wanted to look at the numbers and the ranked-choice voting to decide about what to do in the future, because I'm also on an independent line."

Cuomo was seen as a moderate and the establishment favourite, known across the country after his governorship during the Covid pandemic.

Mamdani is a millennial outsider who was fairly unknown until recently.

Born in Uganda, his family moved to New York City when he was seven. He has posted one campaign video entirely in Urdu and mixed in Bollywood film clips. In another, he speaks Spanish.

Mamdani's strong support of Palestinians and criticism of Israel put him at odds with most of the Democratic establishment.

He went viral during his campaign for videos where he questioned NYC voters who swung for Trump in the November election.

He asked what issues led them to cast their ballots for the Republican president and what it would take for them to swing Democrat.

Mamdani's platform includes free public buses, universal childcare, freezing rent in subsidised units, and city-run grocery stores - all paid for by new taxes on the rich.

"This is a city where one in four of its people are living in poverty, a city where 500,000 kids go to sleep hungry every night," he told the BBC at a recent event.

"And ultimately, it's a city that is in danger of losing that which it makes it so special."

Congresswoman Alexandria Ocasio Cortez and Senator Bernie Sanders, also democratic socialists, endorsed Mamdani during his campaign. (BBC)

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