British survivors of 9/11 were among those who gathered in London's Queen Elizabeth Olympic Park on Friday to mark the 25th anniversary of the attacks in the US.
The event, hosted by UK charity Since 9/11, was held near a memorial sculpture made from steel recovered from the ruins of the World Trade Center.
In a video address played to attendees, Prime Minister Andy Burnham said: "On this anniversary, we must remember that hatred and fear did not have the last word on 9/11."
The Duke and Duchess of Gloucester, with the US ambassador to the UK, have separately attended a special Changing of the Guard ceremony while a number of vigils are planned in the US for the day.
Burnham, whose comments were broadcast on a screen, said: "Time may pass, but I know the grief is still felt just as deeply today.
"But our response was clear: those who seek to destroy our way of life will never win."
Referencing attacks that have taken place in Manchester, the prime minister added: "I will never forget the messages of support and friendship from people across America who knew better than anyone the suffering of our city."
On Tuesday 11 September 2001, suicide attackers hijacked four US passenger jets and crashed them into the Twin Towers of the World Trade Center in New York, which subsequently collapsed, and the Pentagon in Virginia.
The fourth plane crashed in a field outside Shanksville, Pennsylvania, after passengers fought back against the hijackers.
67 British citizens were among the 2,977 people murdered.
Other speakers included Chief Rabbi Ephraim Mirvis who underlined that in Jewish tradition "anniversaries are not so much about the past but rather what we can learn from the past for the sake of our future".
"When confronted by hate, our response must be to increase our love of community, of decency, of fairness, of family," he added.
Cellist Megan Clarke also played for the first responders, school children, family members and others who attended.
One of those attending Friday's event, Liam McKenna, lost his aunt and uncle in the attacks.
He was aged 15 at the time and remembered watching events unfold on TV.
He told the BBC how important it is for him and his family not only to never forget what happened that day - but also to ensure future generations learn about the events of 25 years ago.
Others have similarly issued tributes, including leader of the Liberal Democrats Ed Davey who said he still feels an "overwhelming mix of shock, grief and outrage".
"We stand united against terror, and in defence of freedom."
Wales' First Minister Rhun ap Iorwerth covered the aftermath of the attacks as a journalist.
"I remember arriving and it being a distinctly different city," he said.
"The smell, I remember still - the burning.
"This bold city, quieter. I remember the sense of shock, which was palpable on the streets of New York."
Conservative party leader Kemi Badenoch said 9/11 was "a terrible day for not just the US, for the UK and the whole world".
"The world changed that day. Life changed for so many of us. Even the conflicts that we see today, a lot of them have roots in what happened back on September 11th."
Vanessa Lawrence was taking part in an artists' residency on the 91st Floor of the North Tower at the World Trade Center in New York when the attack happened on September 11 2001 and managed to escape.
She told BBC Radio Scotland's Breakfast programme on Friday morning: "At the point I came out of the building, I remember sort of silence and slow motion, but it was obviously very loud and somebody made me look up and that was the big dust cloud coming down, so that was the South Tower collapsing, which I got caught under â luckily not hit by anything."
