The terrorists behind the 9/11 attacks in the United States failed to shake the belief in freedom and democracy, British Prime Minister Boris Johnson said on Saturday, as the world marked the 20th anniversary of the terror strikes.

In a message on Twitter, Johnson said that while the terror threat may not have gone away, people have refused to live in permanent fear".

Today we remember the 2,977 people taken from us on September 11th 2001, said Johnson in his message.

But while the terrorists imposed their burden of grief and suffering, we can now say with the perspective of 20 years that they failed to shake our belief in freedom and democracy, he said.

"That we are coming together today in sorrow but also in faith and resolve demonstrates the failure of terrorism," he said.

A total of 2,977 people died in the terror attacks in New York and Washington, including 67 Britons.

The attacks, which were planned by Al Qaeda from Afghanistan, saw four US passenger jets seized by suicide attackers two of which were flown into the Twin Towers of the World Trade Center in New York.

Another plane crashed into the Pentagon, just outside the US capital, Washington DC, and a fourth plane crashed in a field in Pennsylvania.

Events are being held around the world to remember those who lost their lives. As part of the day's events to mark the anniversary, a private service of remembrance organised by the September 11 UK Families Support Group will be held at Grosvenor Square in central London.

After sunset, 67 candles will be lit in the garden to remember each of the British victims.

Johnson's address to be played at a memorial event at the Olympic Park in east London on Saturday refers to recent events in Afghanistan, which had only strengthened people's belief in freedom and democracy.

He said: "Twenty years ago, September 11, 2001 became, in President (Franklin) Roosevelt's words after Pearl Harbor, a 'date which will live in infamy'.

"But while the terrorists imposed their burden of grief and suffering, and while the threat persists today, we can now say with the perspective of 20 years that they failed to shake our belief in freedom and democracy; they failed to drive our nations apart, or cause us to abandon our values, or to live in permanent fear."