NEW YORK, United States (AFP) — Zohran Mamdani, the young star of the United States (US) left, started his first day as New York mayor on Thursday for a term sure to see him cross swords with President Donald Trump.

The 34-year-old Democrat — virtually unknown a year ago — was officially sworn in as mayor of the largest city in the United States just past midnight as New Yorkers rang in the new year.

New York’s first Muslim mayor took the oath of office during a private ceremony at a decommissioned subway stop under City Hall.

Later on Thursday, Mamdani is scheduled to take part in a larger, ceremonial inauguration 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.

“This is truly the honour and the privilege of a lifetime,” Mamdani told reporters after taking the official oath in the first minutes of New Year’s Day.

But it remains to be seen if Mamdani, a self-described democratic socialist, can deliver on his ambitious agenda, which envisions rent freezes, universal childcare and free public buses.

Once an election is over, “symbolism only goes so far with voters. Results begin to matter a whole lot more,” New York University lecturer John Kane said.

How Trump behaves could be decisive.

The Republican, himself a New Yorker, has repeatedly criticised Mamdani, but the pair held surprisingly cordial talks at the White House in November.

Lincoln Mitchell, a political analyst and professor at Columbia University, said the meeting “couldn’t have gone better from Mamdani’s perspective.”

But he warned their relationship could quickly sour.

One flashpoint might be immigration raids as Trump wages an expanding crackdown on migrants across the United States.

Mamdani has vowed to protect immigrant communities.

Before the November vote, the president also threatened to slash federal funding for New York if it picked Mamdani, whom he called a “communist lunatic.”

The mayor has said he believes Trump is a fascist.

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.