Democrat Bill De Blasio squeaks into New York mayoral race

New York's public advocate Bill De Blasio has won the Democratic mayoral primary, unofficial results showed, a springboard to becoming the next mayor of the city.

With 97 per cent of ballots counted across the Big Apple, the 52-year-old had 40.2 per cent of the vote, a hair's breadth above the magic number needed to avoid an October 1 run-off.

His victory followed a bruising campaign overshadowed by the lurid sex scandal embroiling rival Anthony Weiner, who trailed in last.

But given the razor-thin margin, former city financial controller Bill Thompson - in second place with 26.2 per cent - held out hope that De Blasio would drop below the 40 per cent mark and force a run-off.

"We are going to wait for every voice to be heard, every voice to be counted. This is far from over," said Thompson, the sole African-American candidate, who narrowly lost to Michael Bloomberg in 2009.

The Democrat candidate will be seen as most likely to replace billionaire Bloomberg, who has led the city for 12 years, after the November 5 election.