Zohran Mamdani, the young upstart of the US left, was sworn in early on Thursday to take over as New York mayor for a four-year term sure to see him cross swords with US President Donald Trump.

The 34-year-old Democrat took his oath of office at an abandoned subway stop under City Hall just after midnight to take the helm of the United States’ largest city. He will be New York’s first Muslim mayor.

His office said the understated venue beneath City Hall reflected his commitment to working people, after the 34-year-old Democrat campaigned on promises to address the soaring cost of living.

NBC News reported that Mamdani took the oath with his hands on two copies of the Holy Quran — one belonging to his grandfather and the other from the New York Public Library’s Schomburg Centre for Research in Black Culture.

“Happy New Year to New Yorkers, both inside this tunnel and above,” said Mamdani, standing beneath a vaulted ceiling with the words City Hall over his head. “This is truly an honour and a privilege of a lifetime.”

He also called the ornate subway station “a testament to the importance of public transit to the vitality, the health, and the legacy of our city”.

Mamdani’s private swearing-in to start his four-year term was performed by New York Attorney General Letitia James, who successfully prosecuted Trump for fraud. His wife, Rama Duwaji, was also present alongside him.

According to CNN, his parents, filmmaker Mira Nair and Mahmood Mamdani, a professor at Columbia University, were also in attendance.

A larger, ceremonial inauguration is scheduled later on Thursday with speeches from left-wing allies Senator Bernie Sanders and Congresswoman Alexandria Ocasio-Cortez.

Around 4,000 ticketed guests are expected to attend the event outside City Hall.

Mamdani’s team has also organised a block party that it says will enable tens of thousands of people to watch the ceremony at streetside viewing areas along Broadway.

New York law spells out that four-year mayoral terms start on the January 1 after elections. To avoid any ambiguity about who’s in charge of America’s most populous city, it has become a tradition to hold a small swearing-in just after midnight.

Born in Uganda to a family of Indian origin, Mamdani moved to New York at age seven and enjoyed an elite upbringing with only a relatively brief stint in politics, becoming a member of the New York State Assembly before being elected mayor.

He is surrounding himself with seasoned aides recruited from past mayoral administrations and former US president Joe Biden’s government.

Mamdani has also opened dialogue with business leaders, some of whom predicted a massive exodus of wealthy New Yorkers if he won. Real estate leaders have debunked those claims.

He has promised a freeze on rents and free buses and childcare, building a campaign around affordability issues that some have seen as a path forward for his Democratic Party around the country ahead of midterm elections.

Mamdani had inspired a record-breaking turnout of more than two million voters and took 50 per cent, nearly 10 points ahead of Andrew Cuomo running as an independent and well ahead of Republican Curtis Sliwa.