De Blasio, the city's public advocate, held a comfortable lead over former city Comptroller William C. Thompson Jr. (26 percent) and City Council Speaker Christine C. Quinn (15 percent), but it wasn't clear whether he would top the 40-percent threshold needed to become the nominee outright or face a runoff against the runner-up, NBC-TV, New York, reported.
Democratic mayoral hopeful Anthony Weiner, hit hard by revelations he was involved in sexting even after resigning his U.S. House seat over an earlier sexting scandal, trailed with 5 percent of the vote.
Among Republicans vying to succeed Michael R. Bloomberg as mayor of the Big Apple, Joseph J. Lhota, a former Metropolitan Transportation Authority chairman, had 51 percent, and John Catsimatidis, a billionaire businessman, had 42 percent.
The New York Times said less than one-in-four Democrats and Republicans were expected to vote in the party primaries.
The mayoral general election is in November.