Newly elected Mayor of New York Bill de Blasio (R) is sworn in by former President Bill Clinton at City Hall in New York, January 1, 2014. [Photo/Agencies]

Bill de Blasio, an unabashed liberal Democrat who campaigned to reduce the gap between New York City's rich and poor, was formally inaugurated on Wednesday as the city's 109th mayor at a ceremony on the steps of City Hall.

Former US President Bill Clinton administered the oath of office using a Bible once used by Franklin Delano Roosevelt.

De Blasio had been sworn in earlier, just after midnight, at a 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. Bloomberg's policies have been credited with making the city safer, greener and more livable.

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.

Running for office, de Blasio presented himself as an anti-Bloomberg candidate, decrying the "tale of two cities" that he said has emerged as New York shed its reputation, from the 1970s and 1980s, as a gritty and dangerous place.

After a resounding victory in November with more than 70 percent of the vote, de Blasio pledged to confront an affordability gap that has left those in the middle and bottom rungs of the economic ladder struggling to pay for basic services such as housing and mass transit.

"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 in excerpts of his inaugural speech released beforehand.

"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," he said.

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.

De Blasio has made some major promises, including a signature proposal to create universal access to pre-Kindergarten and middle school after-school programs, and his critics are likely to seize quickly on his ability to deliver.

"We will ask the very wealthy to pay a little more in taxes so that we can offer full-day universal pre-K, and after-school programs for every middle school student," de Blasio said in the prepared remarks. "We do not ask more of the wealthy to punish success. We do it to create more success stories."

Those programs depend on approval by state lawmakers and Governor Andrew Cuomo of an income tax increase on the city's highest earners. Cooperation from Albany is far from assured.

De Blasio has also pledged to improve police and community relations to extend New York's historic drop in crime as well as to fight the closing of community hospitals.

While Bloomberg has left the city with no budget deficit for the current fiscal year, contracts for all of the public sector unions have expired.

In a news conference on Tuesday, de Blasio said he hoped to have the new contracts in place within a year.

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 US first lady Hillary Clinton ran for US 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.

On Wednesday, the city's new comptroller, Scott Stringer, and its new public advocate, Letitia James, also were sworn in. Both are Democrats and close allies of de Blasio.