Joe Biden led the six minutes of silence in New York City

Celebrities have paid tribute to the nearly 3,000 lives lost on September 11 2001 as the world mourns the 20th anniversary.

The attacks were a series of four coordinated terrorist attacks and it remains the deadliest terrorist attack in human history, with 2,977 innocent people dying including 340 firefighters and 72 police officers, and over 25,000 injured.

Two planes crashed into the Twin Towers in Manhattan, while a third hit the Pentagon, outside Washington DC. The fourth had an unknown target in the nation's capital but passengers attempted to regain control of the aircraft and it crashed in a field in Pennsylvania.

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Sarah Michelle Gellar was one of many to take to social media to share her memories of that day, writing; "It was one of the worst days in history, but it brought out such kindness and unity. At a time when America is more divided than ever, today let’s remember the thoughtfulness we showed each other. #neverforget that day, but remember the nation we were after."

Kate Hudson added: "It’s hard to believe it’s been so long since this devastating event in our history. Being in the city right now brings back so many memories. Everyone remembers where they were."

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"We were there for each other. We were one. Let’s never forget how much we need each other," wrote Christie Brinkley.

"Today my heart is with those we lost and the families who have missed them and kept their memories alive for 20 years with their love."

Reese Witherspoon shared this moving tribute

Catherine shared two moving images from the day and the following months

Mariska posted a quote from the wife of Flight 93's pilot

Christie Brinkley's posts focused on the first responders who bravely walked into the towers

star Mariska Hargitay simply shared a post that had a quote from the wife of Flight 93 pilot Jason Dahl, which read: "If we learn nothing else from this tragedy, we learn that life is short and there is no time for hate."

Catherine Zeta-Jones, meanwhile, simply paid tribute by posting a devastating picture of the aftermath of the attack with the Statue of Liberty in the foreground, and a stunning shot of Ground Zero with a blue beam of light shooting into the sky.

President Joe Biden and the First Lady Jill joined the Obamas and Clintons at the annual 9/11 Commemoration Ceremony on Saturday, standing together in a moment of silence.

Sarah posted two pictures honoring the horror of the day itself and the message to never forget

Kate shared this tribute focusing on the future

Barack's picture focused on the Ground Zero memorial

They stood for six minutes of silence at times that corresponded with the attacks on the World Trade Center towers as well as on the Pentagon and the crash of Flight 93.

Former President Barack Obama later sent a message, which read, in part: "One thing that became clear on 9/11 — and has been clear ever since — is that America has always been home to heroes who run towards danger in order to do what is right.

"9/11 reminded us how so many Americans give of themselves in extraordinary ways — not just in moments of great crisis, but every single day. Let’s never forget that, and let’s never take them for granted. #NeverForget911."

The Clintons, Obamas, and Bidens at the New York memorial

Britain's Queen Elizabeth also revealed that the American National Anthem will be played at Guard Change at Windsor Castle at 11 am GMT.

Posting on the official Royal Family social media accounts, the message read: "As we mark the 20th anniversary of the terrible attacks on 11th September 2001, my thoughts and prayers – and those of my family and the entire nation – remain with the victims, survivors and families affected, as well as the first responders and rescue workers called to duty.

"My visit to the site of the World Trade Center in 2010 is held fast in my memory. It reminds me that as we honour 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. Elizabeth R."

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