500,000 light bulbs in the world's tallest building, the 828 metre Burj Kalifa tower in Dubai, were turned off. Organisers said six of the world's 10 biggest skyscrapers had registered to plunge into darkness.
Athens the Acropolis was temporarily returned to the shadows of pre-electric times, while Moscow extinguished the floodlights on 70 buildings and bridges. Other great monuments, including Christ the Redeemer statue in Rio de Janeiro, the Eiffel Tower and the London Eye receded into darkness.
In Australia, where polling showed about four in 10 people took part, the Sydney Opera House, Federal Parliament and Federation Square in Melbourne flicked their switches