The global event, organized by environmental group WWF to push for action on climate change and other man-made threats to the planet, sees buildings around the world switch off their lights for 60 minutes at 8:30 pm local time.
In Greece, Athen's Acropolis staged a slow blackout with lights going off one at a time on the Parthenon temple and the other ancient structures on the famous hilltop.
People filmed and cheered as the lights also went out one at a time in Moscow's Red Square and in Paris the iconic Eiffel Tower went dark.
In Berlin, an activist dressed as a giant panda, the symbol of the WWF, switched off the lights on the city's Brandenburg Gate.
The annual Earth Hour has grown steadily since the first event in 2007 in Sydney, Australia and is now observed in more than 180 countries and territories, according to its organisers.