The Duchess of Cornwall applied a traceable liquid called SmartWater to the Royal Artillery Memorial at Hyde Park Corner. The liquid is used to identify memorial plaques that have been stolen.
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Muslim leaders condemned the "heinous and appalling" Westminster terror attack.
Travis Frain is recovering in hospital and has posted a photo to Twitter of Prince Charles by his bedside.
Acting Commissioner Craig Mackey led new policemen and women in paying their respects at a passing-out parade in Hendon, north-west London.