It was unclear whether he had crossed the 40 percent threshold needed to avoid a runoff Oct. 1 with William C. Thompson Jr., who finished second in the Tuesday party primary, The New York Times reported.
Early results indicated de Blasio, New York City's public advocate, won slightly more than 40 percent of the ballots counted but thousands of paper ballots hadn't been counted, election officials said. Counting could take days.
The winner of the Republican contest was Joseph J. Lhota, a former Metropolitan Transportation Authority chairman, who bested billionaire businessman John A. Catsimatidis.
The mayoral election is Nov. 5.