Animated videos will appear in the shadows of people as they walk through Trafalgar Square in one of the world's largest interactive artworks.
Sensors will track movement in the square's north terrace and project moving portraits into the shadows.
Artist Rafael Lozano-Hemmer said the central London square was an ideal location as it was so busy.
The work, entitled Under Scan, was previously tried out in Northampton's Market Square.
Two giant projectors will help pedestrians cast long shadows as part of the installation, in front of the National Portrait Gallery.
Video portraits inside their shadows will then turn to look directly at them and interact, before appearing to lose interest as the person walks away.
Mr Lozano-Hemmer said: "We actually count on people just walking by casting their shadow and all of a sudden you look down and go 'gee - there's somebody inside my shadow and they are waving at me.'"
The installation opens later and will operate from dusk until midnight 30 November.
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