Services of remembrance have been held across the US to honour the nearly 3,000 people who died in the September 11 attacks 15 years ago.
At Ground Zero in New York, hundreds of victims' relatives and dignitaries gathered to hear the reading of the names of the people killed when the Twin Towers were attacked.
US President Barack Obama observed the sombre anniversary with a moment of silence in the Oval Office at 8:46 am (EDT) - the precise moment the attacks began in 2001.
Afterwards, Mr Obama laid a wreath at the Pentagon at the beginning of a memorial service.
In his weekly radio address the President praised Americans for their resilience.
Democratic presidential candidate Hillary Clinton and Republican rival Donald Trump were both at the anniversary ceremony at the World Trade Centre.
Hundreds of people are also attending a ceremony at the Flight 93 National Memorial in Shanksville, Pennsylvania.
The American flag has been lowered to half-staff at the White House and other federal buildings.
Meanwhile in London two one minute silences were held at the 9/11 memorial in Stratford's Queen Elizabeth Olympic Park.
Flowers were also laid at the bottom of the 8.5 metre sculpture, which is the biggest part of World Trade Centre steel outside of America.
It comes from just below the point of impact where the South tower was hit, and comprises three girders held together by a piece of polished metal.
Nearly 3,000 people were killed - 67 of them British - when passenger jets hijacked by al-Qaeda terrorists struck the twin towers of the World Trade Centre in New York and the Pentagon outside Washington.
A fourth hijacked plane crashed into a field in Pennsylvania.