Throughout the world illuminated patches went dark for Earth Hour, a campaign to highlight the threat of climate change.
Time zone by time zone, nearly 4,000 cities and towns in 88 countries joined the event sponsored by the World Wildlife Fund to dim nonessential lights from 8.30pm to 9.30pm.
The campaign began in Australia in 2007 and last year grew to 400 cities worldwide.
Organisers initially worried enthusiasm this year would wane with the world focused on the global economic crisis but they said it apparently had the opposite effect.
UN Secretary Ban Ki-moon called Earth Hour 'a way for the citizens of the world to send a clear message: They want action on climate change'.
An agreement to replace the Kyoto Protocol, which expires in 2012, is supposed to be reached in Copenhagen, Denmark, this December, and environmentalists' sense of urgency has spurred interest in this year's Earth Hour.
Earth Hour organisers said there's no uniform way to measure how much energy is saved worldwide.