US President Barack Obama has said the annual act of remembrance for the victims of the 9/11 attacks provides the chance to recommit the nation's values.
In an address at the Pentagon in Washington, Mr Obama said: "No act of terrorism can change what we stand for."
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Mayor of London Sadiq Khan is backing the scheme.
Sunday will be a fine day across much of Scotland and northern England, with much less coastal mist and low cloud than yesterday.
A spokesperson said the lightning strikes had left the "aircraft fuelling system unavailable."