Hours before greeting King Charles III in Lower Manhattan on Wednesday, Mayor Zohran Mamdani was asked about the royal visit during a news conference in the Bronx.

The mayor quickly distanced himself, saying that he would be attending a ceremony at the 9/11 Memorial and Museum “alongside a number of other elected officials” and that would be the extent of his interaction with the king.

But what would he say to King Charles if they were to have a private meeting?

“I would probably encourage him to return the Koh-i-Noor Diamond,” Mr. Mamdani said.

The mayor’s decision to invoke the 105.6-carat diamond — taken from an Indian prince in the 1840s when India was a British colony and presented to Queen Victoria — was in character.

Mr. Mamdani was born in Uganda, also once part of the British Empire, to Indian parents who have helped to shape his political views. His father, Mahmood Mamdani, is a leading scholar of anticolonialism. His mother, Mira Nair, is an award-winning filmmaker whose movies have examined the colonial experience. India has lobbied for the diamond, part of the crown jewels in the Tower of London, to be returned.

Mr. Mamdani is also the city’s first democratic socialist mayor, and he has said his goal is to uplift the city’s working-class residents.

“The mayor,” said Joe Calvello, his spokesman, “is generally opposed to the idea of a king.”

It’s not the first time that Mr. Mamdani has weighed in on international politics.

The mayor, a staunch opponent of Israel’s war in Gaza and a supporter of Palestinian rights, said before winning the mayoralty that he would call for the Police Department to arrest Prime Minister Benjamin Netanyahu of Israel if he came to New York City. Mr. Mamdani, the city’s first Muslim mayor, angered some Indian American Hindus when he criticized Prime Minister Narendra Modi of India, whom he has also called a “war criminal” for his treatment of the country’s Muslim minority.

Mr. Mamdani did greet King Charles at the memorial, where they exchanged pleasantries and where the king and Queen Camilla left a bouquet of flowers.

At the event, former Mayor Michael R. Bloomberg, a billionaire businessman who is chairman of the museum, was photographed laying flowers with King Charles while the mayor and other elected officials looked on.

“Mike Bloomberg has a deep respect for the monarchy and viewed himself as an ambassador for the city,” said Chris Coffey, a political strategist and onetime adviser to Mr. Bloomberg. “Mayor Mamdani’s politics are a little bit different than that, and he views himself as more of a populist.”

Even though the mayor’s team has made it clear since last week that he had no plans to meet privately with King Charles, not everyone was aware of the limits of their interaction.

On at least one planning document viewed by The New York Times for the king’s visit to Harlem Grown, an urban farming nonprofit in Harlem, “Mayor Mamdani” was listed as a participant in the tour.

Mr. Mamdani did not appear at the event.

Claire Fahy contributed reporting.