11 September marked the 21st anniversary

Sunday marked the 21st anniversary of 9/11, one of the world's deadliest terror attacks. Celebrities across the globe, including Kourtney Kardashian, Kelly Ripa and Tom Brady, marked the poignant occasion with heartfelt tributes on social media.

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Their posts honored the 2,977 innocent victims who died in 2001, including 340 firefighters and 72 police officers, and over 25,000 people who were left injured. The series of coordinated terrorist attacks against the USA saw two planes crash into the Twin Towers in Manhattan, while a third hit the Pentagon, just outside Washington DC.

Kelly Ripa

talk show host Kelly Ripa marked her respect with a poignant photograph shared on her Instagram Story of a brilliant beam of light pouring into the sky from the World Trade Centre memorial in NYC. She simply wrote: "#september11" over the image.

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Nicola Peltz Beckham

Actress Nicola Peltz shared a clip from the NYC Pipe Band marching in the 9/11 Memorial Parade through the city, adding the heartfelt caption: "Never Forget".

The wife of Brooklyn Beckham also reshared a harrowing photograph taken from the moment the second plane hit the Twin Towers of the World Trade Centre, captioned with the date of the unforgettable event.

Kourtney Kardashian

star Kourtney reshared a poignant photograph to her Instagram Story taken by Melanie Einzig. The chilling photograph depicts the moments following the terror attack in New York City.

Demi Moore

Marking her respect, Hollywood actress Demi Moore reshared a photograph from her own Instagram account along with the caption: "Thinking of this beautiful city today [heartbroken emoji] #neverforget."

The raven-haired star is looking out at the NYC skyline in the powerful post.

Carrie Underwood

The country singer re-shared a heartfelt post to her Instagram Story to mark the 21st anniversary of 9/11. The post, originally shared by Tunnel 2 Towers foundation, read: "We paid off the mortgages on the homes of 21 fallen first responder families across the country in honor of the 9/11 heroes who laid down their lives bringing others to safety when America was attacked."

Miranda Lambert

Country singer Miranda Lambert is a passionate animal lover, sharing her $3.4million Tennesee home with her husband Brendan McLoughlin and several animals, including a 400-acre horse farm. Her 9/11 tribute re-sharing a post about the "forgotten" heroes of the attack was a fitting nod to her love for animals.

"September 11th will always be a day of heavy memory. We’d like to take a moment to honor the brave people and dogs who were there on that fateful day. It's estimated that around 300 dogs took part in search and rescue efforts at Ground Zero in the wake of 9/11," read the post.

Hailey Bieber

Model Hailey Bieber kept her tribute short, but powerful. The wife of Justin Bieber added a photo of the former Twin Towers in NYC, along with the caption: "always remember".

Tom Brady

American football quarterback Tom Brady re-shared a powerful photograph of the USA flag on his Instagram Story, along with the caption: "Today & always, we remember 9/11."

Kate Hudson

Fabletics founder and actress Kate Hudson chose not to pay her respect with words, but rather chose to share a stunning photo of the USA flag draped between two cranes against the NYC skyline on her Instagram Story.

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