Former U.S. President Bill Clinton administered the oath of office on a Bible once used by Franklin Delano Roosevelt. The formal midday service was held hours after De Blasio officially took office just after midnight on January 1 in a small ceremony at his home in Brooklyn.
He succeeds Michael Bloomberg, who led the city in the aftermath of the attacks of September 11, 2001 and the recession six years later, and whose policies have been credited with making the city safer, greener and more livable.
De Blasio, who captured more than 70 percent of the vote last November, presented himself as an anti-Bloomberg candidate, decrying the economic inequality that he said has emerged as New York shed its reputation from the 1970s and 1980s as a gritty and dangerous place.
Over the last decade, as the city prospered, apartment rents in New York City rose about 44 percent and the cost of a monthly Metro Card jumped 60 percent.
In his inaugural address, de Blasio adamantly underlined that message.
"When I said we would take dead aim at the tale of two cities, I meant it. And we will do it," de Blasio said. "That mission - our march towards a fairer, more just, more progressive place, our march to keep the promise of New York alive for the next generation - it begins today."
"We won't wait. We'll do it now," de Blasio said as he ticked off his priorities: expanding the city's paid sick leave law, forcing large developers to build more affordable housing, reforming the controversial police tactic of stop-and-frisk that critics say leads to racial profiling, and offering universal access to pre-kindergarten and after-school programs.
Indeed, his critics are likely to seize quickly on de Blasio's ability to deliver on signature proposals.
His pre-kindergarten plan hinges on a tax hike for the city's highest earners to pay for it that must be approved by state lawmakers and Governor Andrew Cuomo. Cooperation from Albany is far from assured.
HAVES AND HAVE-NOTS
Even beyond his ambitious, liberal agenda, de Blasio faces the day-to-day challenges of running a city of more than 8 million voters. He has just a few months to craft his first budget and his administration will be tasked with negotiating new contracts with each of the city's public-sector unions, all of which have been working under long-expired terms.
A major winter storm is due to hit the city on Thursday, perhaps offering de Blasio his first major test.
But while Bloomberg often clashed with the City Council and with more liberal citywide officials, de Blasio will be surrounded by allies. The city's new comptroller, Scott Stringer, and its new public advocate, Letitia James, both echoed de Blasio's theme of confronting inequality.
"The growing gap between the haves and the have-nots undermines our city and tears at the fabric of our democracy," said James, a former city councilwoman who is the first black woman to be elected to citywide office in New York City.
De Blasio began his career in government working under David Dinkins, the city's first black mayor who was elected in 1986 and was the last Democrat to hold the post.
In 2000, when former U.S. first lady Hillary Clinton ran for U.S. senator in New York, de Blasio was her campaign manager.
He went on to serve two terms on the New York City Council and four years ago was elected public advocate - a citywide office with a budget of just $2 million that is generally seen as a springboard for the job of mayor.
Bloomberg, who is leaving City Hall after 12 years, has said he plans to take a two-week vacation in Hawaii and New Zealand with his longtime girlfriend, Diana Taylor.
Then, the billionaire, who has homes in Bermuda and London, has said he will focus on his charitable foundation, Bloomberg Philanthropies, and remain active in public health, gun control and government innovation.
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