The US ambassador has laid a wreath at London's 9/11 Memorial Garden to mark the 25th anniversary of the terror attacks.Memorials are taking place across the globe to mark 25 years since the attacks which claimed the lives of 2,977 people, including 67 Brits.The events of 9/11 shook the world when hijackers took control of four passenger jets, crashing three of them into the World Trade Centre in New York, and the Pentagon in Virginia.The fourth crashed into a Pennsylvanian field after the passengers fought back. In London, America's ambassador to the UK - Warren Stephens - has been at the capital's September 11th Memorial Garden in Grosvenor Square alongside his wife Harriet to pay respects to those killed.Speaking at the memorial, Mr Stephens said: 'Harriet and I want to pay our respects to the 67 UK citizens who were killed on 9/11 25 years ago.'It's hard to believe, it's 25 years ago for me, for us. Talking about it as we walked in and it's still hard to believe what happened.'And it's a tragedy none of us will ever forget.' US Ambassador Warren Stephens and his wife Harriet lays a wreath at London's 9/11 memorial garden A massive explosion in the south tower of the World Trade Centre after a hijacked plane crashed into the skyscraper. The events of 9/11 shook the world when hijackers took control of four passenger jetsHe added that the US will never forget the 'friendship' the British showed Americans after the attack with the 'outpouring of letters and flowers from all over the country'.Mr Stephens said tributes were left across the nation and more than 1,000 remain in the archives of the US embassy in London.As ceremonies are being held on either side of the Atlantic on Friday to mark the sombre milestone, Andy Burnham warned that terror 'still knocks at the door of democracy' and expressed the nation's unity with the US.The Prime Minister said the UK and US 'stand together in memory of those we lost, and in defence of the values we cherish'.He recalled how in the wake of terrorist outrages in Manchester during his time as mayor of the city region, messages of support had been received from across the US, including the children of those killed on September 11, 2001.Meanwhile, Sir Tony Blair, who was prime minister at the time and had to navigate its aftermath and the war on terror, said the anniversary was also a time to 'commemorate the unshakeable and essential bond' between the two countries.Later on Friday, the Duke and Duchess of Gloucester will take the Royal Salute on the Buckingham Palace forecourt, accompanied by Mr Stephens. Mr Stephens and his wife at the memorial garden in Grosvenor Square. Memorials are taking place across the globe to mark 25 years since the attacks which claimed the lives of 2,977 people, including 67 BritsIt was on a bright, cloudless day 25 years ago that 2,977 people were killed when passenger jets hijacked by al Qaida terrorists struck the twin towers of the World Trade Centre in New York, causing both to collapse, and the Pentagon outside Washington, DC.A fourth plane seized by extremists – United Airlines Flight 93 – crashed into a field in rural Pennsylvania after the crew and passengers fought back and so prevented it from hitting another target, thought to be the congressional building, which houses the US legislature.It remains the worst terrorist outrage of all time, claiming the lives of victims from more than 90 nations.In response, the US became the first and only ever country to invoke NATO's 'all for one, and one for all' collective security clause, where members rally to the defence of an ally under attack – a commitment now called into doubt by the current president.It led to the large-scale deployment of UK forces to conflicts in both Afghanistan and Iraq, which left hundreds of British troops dead, thousands more injured and continues to have global repercussions.At a ceremony to dedicate a memorial stone to the victims of 9/11 at the US embassy in London on Thursday, Mr Stephens said Britain had rallied in support 'in one of America's darkest hours' and that 'despite our disagreements and debates', the transatlantic relationship 'has kept us safe for centuries'.His comments follow strained relations with President Donald Trump's White House, most notably over the Iran war, which have tested the UK and US' so-called special relationship.Marking the quarter of a century anniversary, Mr Burnham marked the shared values of the UK and US.'On the 25th anniversary of 9/11, we remember the nearly 3,000 people who were killed, including 67 Britons,' he said.'My thoughts are with all those who lost friends and family members that terrible day. Time may pass, but I know the grief is still felt just as deeply today.'On this anniversary, we must remember that hatred and fear did not have the last word on 9/11.'The terrorists sought to weaken and divide America, and everyone who believes in the values our countries share. But our response was clear: those who seek to destroy our way of life will never win.'READ MORE: PETER HITCHENS: The nightmare of 9/11 became a pretext for a new age of repression, stupid wars and censorship. How I miss the freedom, light, joy and humour that existed before. But will we ever get them back? I see no sign of it READ MORE: I was the only Daily Mail reporter in New York when planes hit the Twin Towers… I remember lying in my West Village apartment thinking 'we're all going to die here'