In a message to U.S. President Joe Biden, the British monarch said a 2010 visit to the site of the World Trade Center remains etched in her memory.
"It reminds me that as we honor those from many nations, faiths and backgrounds who lost their lives, we also pay tribute to the resilience and determination of the communities who joined together to rebuild,” she said in a statement.
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Echoing her sentiments, British Prime Minister Boris Johnson, who was born in New York, said the terrorists had "failed to shake our belief in freedom and democracy."
Across the English Channel, French President Emmanuel Macron posted a video of the American flag to Twitter, alongside the caption: "We will #NeverForget. We will always fight for freedom."
European Commission President Ursula von der Leyen also tweeted that the E.U. stood "with the USA in defending freedom & compassion over hate."
"Even in the darkest, most trying of times, the very best of human nature can shine through," she wrote.
South Korean President Moon Jae-in also expressed his “deepest sympathies” to Biden and the American people, writing on Twitter that his country would continue to support U.S. efforts to combat terrorism.
Indian Prime Minister Narendra Modi also used social media to express his condolences. He tweeted that Sept. 11 is a date in history that "struck humanity" and "taught the world a lot."
Elsewhere, in a blog post, Australian Prime Minister Scott Morrison said the events that day were "an awful human tragedy, causing great personal suffering and grief to so many."
He added it "reminds us that we can never take our peace, our freedom and our way of life for granted."
The tributes came in as several events were held in the U.S. to commemorate the 2,977 people who died that day when two planes, American Airlines Flight 11 and United Airlines Flight 175, crashed into New York's World Trade Center twin towers; American Airlines Flight 77 crashed into the Pentagon, in Arlington, Virginia; and United Airlines Flight 93 crashed in field near Shanksville, Pennsylvania.