Television channels including CNN, Fox News and MSNBC projected the Manhattan billionaire the victor in the primary seconds after the polls closed.

US networks said it was too early to call the Democratic primary between former secretary of state and New York senator Hillary Clinton and Brooklyn-born Vermont senator Bernie Sanders, signalling a tighter race.

CNN said Trump won the Republican primary with 68.6 percent of the vote, streets ahead of his evangelical rival Ted Cruz on 13.6 percent and Ohio Governor John Kasich on 17.8 percent, with one percent of the vote counted.

The Empire State Building, one of New York's most iconic landmarks, was lit up in red, the Republican party's color, after the networks called Trump's victory.

Trump's victory on home turf will bolster his quest to sew up the nomination before Republican grandees can swing behind another candidate at the party convention in Cleveland in July. (AFP)