NEW YORK CITY(AFP) - New York exit polls showed that public advocate Bill de Blasio had won the Democratic primary election, a springboard to becoming the next mayor of the overwhelmingly Democratic city.

The left-leaning 52-year-old was reported by local media to have a wide lead over his opponents, however it was not yet clear whether he would secure the 40 per cent needed to avoid a runoff election.

The city's former financial controller Bill Thompson and openly gay city council speaker Christine Quinn, were slugging it out for second place, unofficial results showed.

If no candidate scores an outright victory, a second round of voting will be held on October 1.

Republican Joe Lhota, former public transport chief, is leading against his main opponent, billionaire businessman John Catsimatidis.

If the win is confirmed, Mr De Blasio will face off against the Republican candidate in an November 5 mayoral election.

The Big Apple has been under the bold and brash leadership of Michael Bloomberg, its richest man, for 12 years and many are hoping to usher in a new political era.

While former frontrunner Quinn has been seen as an ally of the mayor, Mr De Blasio has presented himself as the anti-Bloomberg candidate, accusing him of overseeing a widening gap between rich and poor.

He has vowed a tax on those who earn over US$500,000 (S$630,000) to pay for after-school care for young children.