Respects have been paid at a ceremony for Armistice Day at the Cenotaph in Whitehall.
Wreaths were laid at the foot of the monument, following a two-minute silence to remember Britain's war dead.
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The singer had been tweeting in support of migrants when she was bombarded with abusive messages.
Ministers are reportedly considering plans in a bid to lower inward migration.
In a tweet on Saturday, the US president didn't give a reason for his absence but said he wished"everyone well and have a great evening".