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Mr. de Blasio, 51, made his long-expected announcement in front of his home in Park Slope, Brooklyn, surrounded by his wife and teenage son. The modest, homespun tableau represented a calculated contrast to the stentorian formality of a City Hall kickoff to reinforce Mr. de Blasio's pitch as a family man from outside Manhattan, embodying the city's diversity.

Mr. de Blasio outlined several priorities, including improving public education, aiding small-business owners and revamping stop-and-frisk police procedures. And he vowed to be a "mayor for our neighborhoods," and cater to New Yorkers who feel that they have been "ignored and priced out" under Mayor Michael R. Bloomberg, who is in the final year of his third term.

"Let's be honest about where we are today: a city that in too many ways has become a tale of two cities, a place where City Hall too often has catered to the interests of the elite rather than the needs of everyday New Yorkers," Mr. de Blasio said in his prepared remarks.

Recent polls suggest that Mr. de Blasio, despite being elected to citywide office in 2009, is relatively unknown; while his job approval ratings are solid, many voters still do not know enough about him to form an opinion.

Even so, Mr. de Blasio is viewed as one of three top-tier Democrats in the race, with the others being Christine C. Quinn, the City Council speaker, and William C. Thompson Jr., a former city comptroller who lost to Mr. Bloomberg in 2009. Another major Democratic officeholder, John C. Liu, the current comptroller, is also likely to run, but has been hampered by a federal investigation into his campaign finances.

Bruce Berg, a political science professor at Fordham, said that "it's Quinn's race to lose," for now. But he said that Democratic primary elections often skewed toward the most liberal voters, and Mr. de Blasio has "already got a base of credentials upon which he can move as far to the left as he needs to, in order to separate himself from the other Democratic candidates."

The winner of the Democratic primary in the fall is likely to be favored in the November general election because Democrats outnumber Republicans, 6 to 1. Still, no Democrat has won the mayor's office in two decades, and this year's Republican primary has attracted several energetic candidates.

Mr. de Blasio, 51, worked for Mayor David N. Dinkins, then as a regional housing administrator in the Clinton administration under Andrew M. Cuomo, now the governor. He later managed Hillary Rodham Clinton's 2000 campaign for Senate, before winning two terms as a city councilman, starting in 2001.

On the council, he became a passionate voice on homeless and housing issues. As a citywide official, he has been a strong supporter of government transparency and changes in campaign finance rules. He has been a frequent critic of Mr. Bloomberg, especially on issues of policing and income inequality, but has also been one of the mayor's staunchest supporters concerning controversial public health initiatives like such as soda-size restrictions.