Millions were expected to turn off lights and appliances for an hour at 8:30 p.m. Saturday.
This year marks the fifth annual Earth Hour organized by the fund.
In Manhattan, many businesses in Times Square participated as well as the Empire State Building.
The National Cathedral in Washington anticipated again this year. The World Wildlife Fund says others who took part this year included the city of Baltimore, where City Hall lights were dimmed, as well at the National Aquarium.
Four thousand cities in more than 120 countries participated last year in the event that is backed by the United Nations as well as global corporations, nonprofit groups, schools, scientists and celebrities.