Earth Hour, a global environmental initiative to get people thinking about climate change, saw people turning off the lights all over the world for an hour on Saturday evening.
It took place from 8:30 p.m. to 9:30 p.m. local time all over the world.
The World Wildlife Fund said more than a billion people in 100 countries are expected to participate.
The WWF blog reported that Russian cyclists went for a ride with head lamps. "These cyclists are heroes," the group's Russia office said. "In Moscow, it is – 8 [C] degrees."
In Egypt, the Earth Hour word was spread by social media.
Greece and China turned off the lights on famous monuments, including the Great Wall of China, which seemed to make the massive structure disappear. European cities went dark as citizens turned off house lights.