Some 25,000 facilities nationwide, including Tokyo Tower, turned off their lights Sunday evening in an event to help preserve energy.
The Environment Ministry and a nongovernmental environmental organization launched the event two years ago to urge people to switch off.
This year, the event will run until Tuesday, during which participants will turn off the lights for two hours starting at 8 p.m.
On Sunday, people gathered at Zojoji Temple near Tokyo Tower in the dark to enjoy a piano concert lighted by some 1,000 candles.