Donald Trump claims rival Ted Cruz has been "mathematically eliminated"
Democratic frontrunner Hillary Clinton has won a comfortable victory in the primary in her home state of New York.
The former US Secretary of State, who represented New York in the Senate for eight years, held a 16% lead on rival Bernie Sanders with around 70% of precincts reporting.
The victory will see Mrs Clinton attempt to recapture momentum lost to the Vermont Senator - who had won seven of the last eight state nominating contests.
Speaking after the result was announced, Mrs Clinton said "victory is in sight" as she played to her home crowd.
She said: "New Yorkers, you've always had my back, and I've always tried to have yours".
"Under the bright lights of New York, we have seen how it's not enough to diagnose problems - you have to actually explain how you're going to solve them".
Mr Sanders had campaigned aggressively in New York, playing on his roots growing up in Brooklyn.
It was also a good night for Republican frontrunner Donald Trump, who easily won the primary also in his home state - extending his lead over rivals Ted Cruz and John Kasich.
Early indications had the real estate tycoon, who leads the Republican nomination contest and was widely expected to win the state, with around 60% of the votes.
Mr Trump was hoping to secure more than 50% of the vote across New York state, which would see him win all of the 95 delegates up for grabs.
The victory also sets him up for next week, when five other northeastern states hold their nomination contests.
Speaking after the result of the primary was announced, Mr Trump said Texas Senator Ted Cruz was "just about mathematically eliminated" from winning the Republican nomination.
Governor Kasich, who has won only his home state of Ohio, was projected to finish second in New York, according to US media reports.
Mr Cruz, who is Mr Trump's closest rival, downplayed the billionaire's victory as little more than "a politician winning his home state".
Mr Trump needed a strong showing to maintain his chances of winning the Republican nomination before the party's convention in July.
But voting was overshadowed by official confirmation that more than 125,000 people were missing from New York City voter rolls, amid reports of other irregularities.
New York City Comptroller Scott Stringer ordered an audit of the city elections board after it confirmed the names had been removed from voter rolls.
The city has roughly four million voters considered active for the presidential primaries.
Mr Stringer complained to the board that it was "consistently disorganised, chaotic and inefficient", citing faulty ballot scanners, late-opening polling stations and skeleton staffing.
Prior to the New York primary, Mrs Clinton had 1,758 of the 2,383 party convention delegates needed to win the Democratic nomination. Mr Sanders had 1,076.
On the Republican side, Mr Trump had 756 delegates before New York, while Ted Cruz has 559 and John Kasich has 144.