NEW YORK » Bill de Blasio, whose fiery populism propelled his rise from obscure neighborhood official to the 109th mayor of New York, was sworn into office Wednesday, pledging that his ambition for a more humane and equal metropolis would remain undimmed.

In his inaugural address, de Blasio described social inequality as a "quiet crisis" on a par with the other urban cataclysms of the city’s last half-century, from fiscal collapse to crime waves to terrorist attacks, and said income disparity was a struggle no less urgent to confront.

"We are called to put an end to economic and social inequalities that threatened to unravel the city we love," he said to about 5,000 people at the ceremony, many beneath blankets on a numbingly cold day.

De Blasio, 52, the first liberal to lead City Hall in two decades, delivered his critiques as his predecessor, Michael R. Bloomberg, whose Wall Street pedigree and business-first approach to governance seemed to embody the city’s current gilded era, sat unsmiling a few feet away.

It was one of many potent symbols of change that dominated a ceremony unlike many before it.

Gone was the more solemn air of inaugurations past, replaced by the booming strains of disco, soul, and dance music by the Commodores, Marvin Gaye, and Daft Punk, spun by a local DJ stationed high above the audience. (Even Hillary Rodham Clinton, seated onstage, swayed with the music.)

Several of the nation’s pre-eminent Democrats – including Gov. Andrew M. Cuomo and former President Bill Clinton, who administered the oath of office over a Bible once owned by Franklin D. Roosevelt – appeared with de Blasio on the dais, celebrating the elevation of a party stalwart with whom they had close ties.

The ceremony was filled with an unusually open airing of the city’s racial and class tensions, including a poem bristling with frustration about "brownstones and brown skin playing tug-of-war," a pastor’s words about "the plantation called New York," and denunciations of luxury condominiums and trickle-down economics.

De Blasio, a careful custodian of his image, took pains to choreograph the appearance of a newly approachable and inclusive City Hall, arriving with his family on the subway and walking onstage to doo-wop tunes. Even the placement of cameras seemed to ensure that only the dignitaries onstage and ordinary New Yorkers arrayed behind them would be shown – and not the many lobbyists and political operatives in the crowd.

And although he warned that his administration’s work "won’t be easy," de Blasio made only passing reference to the myriad and daunting challenges – fiscal, political and structural – that he will face in enacting his ambitious policy agenda.

Several of his proposals, including his signature plan to pay for prekindergarten classes by raising taxes on the wealthy, are at the mercy of the governor and state legislators in Albany. Other elements of his platform are expected to be opposed by powerful interests in the city’s corporate classes.

But in his first hours as mayor, de Blasio opted to focus more on his aspirations for the office, and fulfilling a campaign promise to change the tone of city government on Day One.

The mayor’s transition team held a ticket lottery so that ordinary New Yorkers could attend the inaugural ceremony, and the City Hall plaza was filled with a diverse crowd that punctuated speeches with impromptu cheers, lending the feel of a jamboree to an event typically more formal than festive.

From her seat in a back row, Justina Taylor, 16, of the Bronx, started singing along with a Jay Z song. "This is my kind of inauguration," she said.Light moments abounded.

The young children of Scott M. Stringer, who was being sworn in as the city comptroller, squealed as their father sought to recite the oath of office and drowned out his words. Stringer laughed: "Guess he’s not quite ready for the television commercial," he quipped – a sly reference to the celebrity that de Blasio’s 16-year-old son, Dante, attained after starring in his father’s campaign ads.

De Blasio, clad in a black topcoat and red-and-white striped tie, was first glimpsed on a stadium-size screen emerging from a nearby subway station, as the song "Boy From New York City" played over loudspeakers. "He’s kind of tall," went one lyric, a fitting description for de Blasio, who, at 6-foot-5, is the tallest to hold the office in at least a generation.

The event turned emotional at times, as well.

The crowd was rapt as Ramya Ramana, a young Indian-American poet, made a cri de coeur in verses addressing familiar themes of class and poverty from de Blasio’s campaign. Ramana, in an appearance partly arranged by McCray, a poet herself, described a New York that was "not lights, not Broadway, not Times Square," but "coffee-colored children playing hopscotch on what is left of a sidewalk."

Letitia James, the new public advocate, delivered what amounted to a blistering rebuke of Bloomberg’s 12-year tenure as mayor. The first minority woman to hold a citywide elected office, James invoked images of "decrepit homeless shelters" in the "shadow of gleaming multimillion-dollar condos."

To underscore her message, James invited Dasani Coates, a young homeless girl featured in a recent series in The New York Times, to stand by her side. Dasani, who sat onstage behind de Blasio and his family, also held the Bible for James as she took the oath of office. If Bloomberg was perturbed by the tone of the proceedings, he did not let on. He sat stoically throughout, his lips pursed. Now out of office, he was headed to Hawaii and New Zealand for a two-week vacation.

In his own speech, which lasted 19 minutes, de Blasio urged the crowd to acknowledge Bloomberg’s work in public health and environmental policy, which he called "a noble legacy."

But de Blasio did not hesitate to restate his determination to change the Police Department’s stop-and-frisk tactics, saying that he wished to "protect the dignity and rights of young men of color."

And he invoked the names of towering liberals in New York’s past, including former Gov. Alfred E. Smith and former Mayor Fiorello H. La Guardia as left-leaning politicians who proved that social reform was possible in a city often synonymous with unfeeling, free-market capitalism.

After a highly personal campaign in which he placed his family at the forefront of advertisements and pamphlets, de Blasio unsurprisingly made sure that his wife and children held central roles at the event. He introduced McCray as "my partner in all I do."

And after the singer Harry Belafonte opened the ceremony, Dante de Blasio stood and slowly guided the 86-year-old Belafonte back to his seat.

The political royalty on hand included Sen. Charles E. Schumer and former Mayor David N. Dinkins, who provided de Blasio with his first professional experience in City Hall, and with a colleague who later became his wife. Former Mayor Rudolph W. Giuliani did not attend. Edward I. Koch, who died last year and supported a rival of de Blasio’s in the mayoral race, went unmentioned.

After the ceremony, de Blasio greeted those who had attended inside City Hall on a receiving line that lasted some three hours, part of a conspicuous effort to show that city government would be more open during his tenure.

But his final event of Wednesday would be a private one: The mayor was set to host a party at Gracie Mansion, his new residence, for colleagues and friends.

The public was not invited.