New York City's public advocate holds a clear lead in the Democratic primary election for mayor as polls closed, according to early and incomplete voting returns.
It was unclear, though, whether he would top the 40% threshold needed to avoid a run-off.
With 94% of precincts reporting, Mr de Blasio, 52, has about 40.02% of the total vote. He needs to stay above 40% in order to avoid triggering an automatic run-ff on October 1. If he cannot, he will face former city Comptroller Bill Thompson, who has 26%.
Council Speaker Christine Quinn was third on 15%, followed by current city Comptroller John Liu with 7%. Elections officials are expected to count an additional 30,000 or more votes in coming days as absentee ballots arrive by post and paperwork comes in from voters who had problems at the polls. A final result may not be known for 10 days.
The winner of the mayor's race in November will assume the helm of the nation's largest city at a critical juncture, as it experiences shrinking crime rates yet widening income inequality, and as the nearly completed One World Trade Centre building symbolises a new era after the terrorist attacks of 2001.
The exit polling showed the appeal of Mr de Blasio, the city's elected public advocate, to be broad-based - he was ahead in all five boroughs; was ahead of Ms Quinn, the lone woman in the race; and ahead of Mr Thompson, the only African-American candidate. The voter interviews were conducted by Edison Media Research for The Associated Press and other news organisations.
If no candidate surpasses 40% of the vote, the top two finishers advance to a run-off.
The winner of that contest would face the Republican nominee in the general election on November 5. Joe Lhota, ex-transit authority chairman and former deputy mayor to Rudolph Giuliani, was battling billionaire grocery magnate John Catsimatidis for the Republican nomination. Exit polling was not available in that race.
In the closely watched race for New York City comptroller, Manhattan Borough President Stringer defeated ex-Governor Eliot Spitzer, who was seeking a return to politics after resigning from New York's governor's office in 2008 amid a prostitution scandal.
Mr Bloomberg, the businessman Republican-turned-independent, is completing his third term. While the city's registered Democrats outnumber Republicans six to one, the Republican Party's recent success in mayoral elections has been largely attributed to a crime epidemic, the aftermath of the Sept. 11 attacks or other extraordinary circumstances.