People across the globe have been sharing moving tributes to commemorate the unforgettable horror of the September 11, 2001 terror attacks.
In remembrance, politicians and high-profile figures have paid tribute to the 2977 people killed in the unimaginable terror of 9/11 - the deadliest terrorist attack in human history.
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Veteran reporter Katie Couric delivered the news that day on the 'Today' show.
"8:46am - the exact time that the first tower was hit. 2,977 people killed, more than 6,000 people injured," she wrote on Twitter.
"Countless families ripped apart, too many broken hearts to imagine, a country forever changed... remembering those we lost on 9/11.
"I will always remember."
Actress Kate Hudson composed a lengthy message on her Instagram account.
"Remembering this day 18 years ago... I was living in New York at the time and I will never forget the way every person in the city supported each other in such an intense time of mourning," she wrote.
Reese Witherspoon paid tribute to the men and women who risked their lives for the rescue and recovery effort.
"We will #neverforget and continue to pay tribute to the brave men and women who fight for our freedom every day," she tweeted.
Of the 2,977 victims killed in the September 11 attacks, more than 400 were New York emergency workers who responded to the World Trade Centre. The New York City Fire Department lost 343 firefighters that day. Seventy-one law enforcement officers were also killed.
9/11 is the deadliest incident for firefighters and law enforcement officers in US history.
Arnold Schwarzenegger wrote of his moving visit to New York's first responders less than a month later.
"They were exhausted and they were in mourning, but they just kept going because they were serving a cause bigger than themselves," he posted on Instagram.
"We pledge to honor our history, to treasure our liberty, to uplift our communities, to live up to our values, to prove worthy of our heroes, above all, to never forget," US President Donald Trump tweeted following a sombre ceremony at the Pentagon in Virginia.
The Pentagon in Virginia was hit by a third plane at 9:37am, leading to a partial collapse of the building's west side and the deaths of 125 people inside.
2020 election candidate Bernie Sanders marked 9/11 by reflecting on "the bravery of the emergency workers" and "the 18-year 'war on terror' and the harm it has caused".
Elizabeth Warren also paid tribute to "the courage of the first responders, service members and citizens who risked their lives that day".
"We will always remember them," she tweeted.
US Vice President Mike Pence remembered those who died on the fourth plane, United Airlines Flight 93. Its passengers attempted to regain control from the hijackers, but in the struggle, the plane crashed into a field near Shanksville, Pennsylvania. There were no survivors.
"The American people will forever be inspired by the courage and resolve of everyone of Flight 93... the men and women have earned their place in American history... as long as America endures, we'll tell their story... God bless our beloved fallen," Pence tweeted.
Former President Barack Obama remembered "the spirit of unity and togetherness that defined the weeks and months after".
"On this day of service, I'm reflecting on all those still working to embrace that spirit, today and every day," he wrote.
Newshub.
