It's the first trip to the Big Apple by a reigning British monarch since Queen Elizabeth II visited in 2010.
The king and queen arrived by motorcade and were greeted by various dignitaries at the memorial. They were accompanied by former New York City Mayor Michael Bloomberg while walking to one of the parapets ringing the two pools, bearing the names of the victims of the attacks. Charles laid flowers on the parapet.
The dignity of the moment meant a great deal to Englishman Sebastian Young-Slee and his American wife, Savannah.
They couldn't see much from their vantage point on West Street, but the memory of this day provides a special gift as they celebrate their first anniversary on Friday.
"I think it's beautiful. I mean, amongst and amidst everything that is going on in the world, a special relationship that's been special for many years. I think it shows that there is still that tie, there still is that link," Sebastian Young-Slee said.
"I think it's really beautiful. I have never had any interaction with the royals at all, so being able to have them over here in the States is phenomenal, and especially commemorating such a monumental moment in our history," Savannah Young-Slee added.
The royal couple then shook hands and spoke to a line of people, including families of victims of the Sept. 11 attacks and first responders. Some relatives held up photos of their lost relatives. Also in the line were New York City Mayor Zohran Mamdani, New York Gov. Kathy Hochul, New Jersey Gov. Mikie Sherrill and other dignitaries. The ceremony came ahead of the 25th anniversary of the attacks.
A small crowd gathered across the street from the memorial site, with views hampered by tight security and a heavy police presence that restricted access and temporarily shut down traffic.
Nonetheless, for people like Lucy Kuhn, who lives nearby in Battery Park City, this was the place to be.
"We've come down and seen the Pope, several popes, and I was just hoping to see him. Maybe you'll see him on the news tonight. I think I'll get a view from the living room" she said.
Before leaving, Charles and Camilla did not make any public remarks at the memorial, in keeping with tradition. The memorial plaza has generally been off-limits to speeches by politicians or other public figures, with the annual commemoration ceremonies there featuring remarks only from the families and friends of the victims.
The stop was part of a busy swing through the city midway through the royal couple's four-day trip to the U.S. to mark 250 years of American independence.
Among the crowd at the memorial were New York Gov. Kathy Hochul, New Jersey Gov. Mikie Sherrill and New York City Mayor Zohran Mamdani.
Mamdani - who was born in Uganda to parents from India, both former parts of the British Empire - shook hands with the king, and the two appeared to greet each other warmly. They spoke only for a few seconds.
Earlier in the day, Mamdani said he hoped to keep the event's focus on 9/11 victims and not pursue a political conversation. But pressed by a reporter, Mamdani had said that if he were to speak with Charles under different circumstances, "I would probably encourage him to return the Koh-i-Noor diamond."
The gem, one of the largest cut diamonds in the world, is part of the Crown Jewels. Seized by the East India Co. after the Second Anglo-Sikh War of 1849, it was given to Queen Victoria and is on display in the Tower of London. Countries including India, Pakistan, Iran and Afghanistan have claimed ownership.
More events planned for the king and queen
From there, Queen Camilla stopped at the New York Public Library, where she delivered a new Roo doll to add to the library's famed collection of Winnie-the-Pooh stuffed animals.
The queen chatted with actress Sarah Jessica Parker during a walk through the building as a crowd of onlookers watched from across Fifth Avenue.
Camilla delivered a new Roo doll to add to the library's famed collection of Winnie-the-Pooh stuffed animals, as the beloved children's character turns 100 this year.
The five dolls currently on display - Winnie-the-Pooh, Piglet, Tigger, Eeyore and Kanga - were the inspiration for the characters in A.A. Milne's children's books. They were owned by the English author's son, the real-life Christopher Robin, in the 1920s. The dolls were donated to the library in 1987 and are a centerpiece of the library's collection of children's literature. Roo, in the books, was a small brown kangaroo and the son of Kanga.
But the uncontested highlight for a group of lucky public-school children was the reading the queen did for them from an excerpt of Winnie-the-Pooh.
Meanwhile, King Charles traveled up town to 134th Street, where he met with the nonprofit, Harlem Grown, which provides free locally grown fruits and vegetables to the community. The king's interest in environmental initiatives is well known.
At Harlem Grown's 134th Street Farm, he planted lavender and mustard seeds with children, saw a chicken coop and watched a live food demonstration that educated children about food and nutrition.
The king and queen later met at Rockefeller Center with economic leaders from both sides of the pond, as promoting economic partnerships is an important goal of this royal visit.
They took part in another event celebrating cultural partnerships between the two countries.
In their last public event of the day in New York on Wednesday, the king and queen attended a gala reception at Rockefeller Center.
Singer and songwriter Lionel Richie, who has worked with the King's Trust for four decades, introduced the royal. Also present were Vogue editor-in-chief Anna Wintour, lifestyle icon Martha Stewart and fashion designer Donatella Versace.
The king closed his brief remarks in a packed wing of the gallery space by joking that he was disappointed he wouldn't get to hear Richie sing.
"I don't know how he does it. He must gargle with port or something."
This is King Charles' 20th visit to the U.S., and his first as king. The trip was planned months ago, despite awkward timing, with President Donald Trump recently slamming the British prime minister for staying out of the war in Iran. But the monarchy is used to staying out of political division at home and abroad, and this visit is meant to reinforce economic, environmental and cultural connections behind that alliance.
The Associated Press Contributed to this report.
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