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.
Other speakers at the Olympic Park 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.