Before meeting King Charles III and Queen Camilla, Mamdani said he would encourage the British monarch to return the Koh-i-Noor diamond.

"And the focus of that wreath-laying is to honour the more than 3,000 New Yorkers who were killed in the horrific terror attacks of September 11. And that's what I'm really looking to do at that event," Mamdani said at a press conference.

He further said, "If I was to speak to the king separately from that, I would probably encourage him to return the Koh-i-Noor diamond."

Following his response, The New York Post Editorial Board called him out saying, "Of course Zohran Mamdani couldn't manage a decent welcome for the king of England: That would've required maturity, grace and humility that our mayor just doesn't have."

"Showing respect for the king is showing respect for Britain, yet Mamdani couldn't resist showing disrespect," the editorial said as it cited previous occasions when members of the British Royal Family were warmly welcomed by NYC mayors, including a "ticker-tape parade" organised in honour of Queen Elizabeth's arrival in the city in 1957 by then Mayor Robert Wagner.

"But Mamdani couldn't bother with any such gestures; he couldn't even set aside his obsession with colonialism, telling reporters he hoped to push Charles 'to return the Koh-i-Noor diamond', a 105-carat gem originally from India and now on display at the Tower of London," the editorial said.

"Mamdani's inexperience is no excuse for letting his personal passions blind him to the duties of his high office," the editorial said, calling out his "addiction to performative displays".

"At best, his youth means there's a chance he'll learn from his graceless mistakes, and do a bit better the next time he's called to represent all the people of New York City," the editorial added.

Another article in the New York Post said that Mamdani had acted "like a petulant teen" in front of the king and queen.On Mamdani's comment about the Koh-i-Noor diamond, the article said that the king's wish to honour the victims of the 9/11 terror attack "was not an opportunity for a freshman barb on colonialism, and to imply that this crown jewel belongs to India".

"Anyway, what is Mamdani's standing to demand the diamond - has he appointed himself a representative of India's government? It wasn't just the mayor's rudeness, but his tone and demeanour. Mamdani did his best version of a petulant teen betraying intolerable boredom," the NY Post article added.

King Charles and Queen Camilla visited the 9/11 Memorial in the city and paid their respects to the victims of the terror attack.

(With Reuters inputs)