People gathered in silence in London to mark the 15th anniversary of the September 11 terror attacks.
Two silences lasting one minute were held at the 9/11 memorial in Stratford's Queen Elizabeth Olympic Park to mark the times two passenger jets hit the World Trade Centre on September 11 2001.
Flowers had been 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.
Peter Rosengard, of the Since 9/11 charity which organised the gathering, said the horror of that day "changed the world forever".
He said: "This steel was one of the pieces in that devastation. It was dragged from the ruins of the World Trade Centre and is now here proudly standing tall in London, as we remember that day.
"It changed our world forever, and now on the 15th anniversary, we remember those innocent victims. We remember and reflect and honour their memory."
Nearly 3,000 people were killed - 67 of them British - when passenger jets hijacked by al Qaida 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.
Mr Rosengard and his daughter Lily, 21, brought the steel back from the United States in 2011 for the 10th anniversary of the attacks.
It is thought to be the only piece to be donated to a capital city.
Designed by American artist Miya Ando, the structure was unveiled by Boris Johnson in the Stratford park in March 2015.
Mr Rosengard continued: "Everything they tried to destroy, those murderers and terrorists, the beating, pulsing heart of the metropolis, is reflected in the sky.
"This remarkable sky today, quite remarkable because you will remember, if you cast your mind back, the incredible blue sky of September 11 2001."
He added: "We polished this steel as a symbol of hope for the future. We polished it to reflect the life, the birds, the trees, children's faces, our faces - everything those terrorists wanted to destroy.
"They didn't; we put the steel up again, here it is."