Mayor Zohran Mamdani will govern “expansively and audaciously,” he said — vowing to stick to his sweeping socialist agenda and bring back “the era of big government” in a fiery speech at his public inauguration ceremony Thursday.

The far-left pol — who went from a backbencher Queens state assemblyman to the most powerful official in New York City — defiantly stood by his promise to tax the rich as he addressed several thousand onlookers from the steps of City Hall.

“I was elected as a democratic socialist and I will govern as a democratic socialist,” he told the packed crowd of several thousand on a frigid New Year’s Day.

“I will not abandon my principles for fear of being deemed radical.”

Mamdani, 34, said he would refuse to “reset expectations” for what the government should do for its citizens, repeating his campaign promises to provide free buses, universal childcare and a rent-freeze on rent-stabilized apartments. 

“The only expectation I seek to reset is that of small expectations,” he said during his roughly 30-minute remarks.  

“Beginning today, we will govern expansively and audaciously.”

The new mayor’s road to election victory — beating establishment figures, including former Gov. Andrew Cuomo — was paved by offering up the more than $10 billion freebie-filled agenda, which he claimed would be paid for by hiking taxes on the ultra-wealthy and corporations.

“The cost of childcare will no longer discourage young adults from starting a family — because we will deliver universal childcare for the many by taxing the wealthiest few,” he vowed in front of Gov. Kathy Hochul, who will need to approve the tax hikes along with the state Legislature.

Follow The Post’s live coverage of New York Mayor Zohran Mamdani’s inauguration

The rallying call came after Vermont Sen. Bernie Sanders, a fellow socialist, delivered the ceremonial oath of office to Mamdani — who was officially sworn in as New York City’s 112th mayor by state Attorney General Letitia James hours earlier, during a private ceremony inside the Old City Hall subway station.

A chant of “tax the rich” broke out from the crowd of supporters gathered in Lower Manhattan — which included celebrities such as Mandy Patinkin, Kal Penn and Richard Kind — as Sanders spoke.

“At a time in our country’s history when we are seeing too much hatred, too much divisiveness and too much injustice, thank you for electing Zohran Mamdani as your mayor,” Sanders said, before arguing that Mamdani’s agenda wasn’t all that “radical.”

“You know, all of us have heard that Zohran’s opponents have called the agenda that he campaigned on radical, communistic. Oh, and absolutely unachievable,” he said. “Really, that’s not what we believe. In the richest country in the history of the world, making sure that people can live in affordable housing is not radical.”

Lefty darling Rep. Alexandria Ocasio Cortez, who endorsed Mamdani’s campaign, delivered opening remarks that also buoyed the mayor’s socialist plans.

Ocasio-Cortez, who represents parts of Queens and the Bronx, argued that voters in November had “chosen courage over fear.”

“We have chosen prosperity for the many over spoils for the few,” she railed. “We have chosen that over the distractions of bigotry and the barbarism of extreme income inequality.”

Mamdani sought to strike a more unifying tone in his address, after taking an adversarial stand during the campaign and immediately after his election,  including bashing Israel and spewing insults toward President Trump.

“This will not be a tale of one city, governed only by the one percent. Nor will it be a tale of two cities, the rich versus the poor,” said Mamdani — who arrived at the event, billed as an “Inauguration for a New Era Block Party,” with his wife Rama Duwaji in a yellow cab.

“It will be a tale of eight and a half million cities, each of them a New Yorker with hopes and fears, each a universe, each of them woven together.”

He said he was ready to be the mayor for voters that “view this administration with distrust or disdain, or who see politics as permanently broken.”

A sign of the divisive positions came out during his speech when a small group of protesters showed up to voice their opposition while waving Israeli and American flags across the street from City Hall.

Some boos also broke out in the crowd when Mamdani mentioned his predecessor Eric Adams, who was in attendance for the ceremony. Mamdani did not criticize Trump in his more than 3,000 word speech.

But he did have a message for residents that don’t want too much government involved in their lives.

“To those who insist that the era of big government is over, hear me when I say this—no longer will City Hall hesitate to use its power to improve New Yorkers’ lives,” he said.

Mamdani admitted during his speech that past mayors have also taken office on similar ideals to his — with some achieving “more success than others.”

“But they were unified by a shared belief that New York could belong to more than just a privileged few,” he said. “It could belong to those who operate our subways and rake our parks, those who feed us biryani and beef patties, picanha and pastrami on rye. And they knew that this belief could be made true if only government dared to work hardest for those who work hardest.

“Over the years to come, my administration will resurrect that legacy.”

— Additional reporting by Hannah Fierick, Matthew Fischettiand Steven Vago