Some of the city's most famous landmarks were plunged into darkness on Thursday night for an hour to highlight climate change.
The normally blazing advertising hoardings at Piccadilly Circus were dimmed...
... while the landmark Tower Bridge took on a menacing shape in the darkness.
Even MPs took part - the Palace of Westminster and St Stephen's Tower were in darkness
Mayor Ken Livingstone's HQ City Hall was unrecognisable as it joined the shutdown.
Londoner Chris Constantine snapped this shot. An hour's "lights-out" in the capital could save enough power to run 3,000 televisions.