The incident unfolded this morning in Oxford Street, in central London.
Scott Collins, who was at the scene, wrote on Twitter: “Blood everywhere on pavement.”
The Metropolitan Police said in a statement: “Police are dealing after a man was injured in a fall.
“Officers were called at 06:30hrs on Tuesday, 14 November to Oxford Street, W1 near Harewood Place.
“A man in his mid 40s, believed to be a window-cleaner, had fallen from a height. London Ambulance Service and London’s Air Ambulance attended the scene.
“The man was taken to a central London hospital in a serious condition. Next of kin have been informed.
“The Heath and Safety Executive has been informed. Roads are closed while emergency services deal with the situation.”