Before the Duchess of Cambridge arrived at Mons Barracks, a soldier had fainted during the St Patrick's Day parade. He was helped to his feet and appeared to be unhurt.
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Local media said the location of the crash was among the most dangerous junctions in the region.
London has welcomed in the Chinese New Year in style with the biggest party outside of Asia.
Detectives have launched a murder investigation after a man was fatally stabbed in Kensington.